**3. Laser beam propagation in atmosphere**

When laser beam propagates in the atmosphere [Tatarskii 1971][Dayton 1992][ Hufnagel 1974][Friend 1967][Andrew 1998], its phase is fluctuated by the scatter of cloud, fog and rain, also by the change of local refractive index of the atmosphere. The fluctuation is varied depending on medium, and it becomes the largest when its size is similar to the size of light, also the medium constant changes spatially and temporally at random. When refractive index slowly and continuously changes due to atmospheric turbulence, the forward scattering dominates because the scale of fluctuation is larger than the wave length. In this case, it is possible to ignore the fluctuation of plane of polarization, but the fluctuation of phase/angle of arrival and the amplitude and the scintillation should be taken into consideration. On the other hand, when the medium is distributed discretely such as cloud, fog and rain and so on, the refractive index changes drastically. Therefore, the equivalent damping which occurs due to absorbing/scattering of particles and the influence against polarization property by multiple scattering with multiple particles should be considered. Modified von Karman spectrum density function is used not to overflow an integral value in theoretical analyses [Andrew 1995]:

$$\Phi\_n(K) = 0.033 C\_n^2 (K^2 + \kappa\_o^2)^{-\frac{11}{6}} \exp\left(-\frac{K^2}{\kappa\_i^2}\right) \tag{1}$$

Herein, 00 0 5.92 / , 2 / *<sup>i</sup>* κ κπ= = *l L*

0 *l* is the inner scale, 0 *L* is the outer scale. Based on the actual measurements of the change of <sup>2</sup> *Cn* (refractive index structure constant) in high altitude, Hufnagel-Valley model (H-V5/7 model) parameterized by the scaling factors of wind speed in upper layer of the atmosphere and the size of the fluctuation of the atmosphere are used:

$$C\_n^2(h) = 0.00594 \left(\frac{\nu}{27}\right)^2 \left(10^{-5} h\right)^{10} \exp\left(-\frac{h}{1000}\right) + 2.7 \times 10^{-16} \exp\left(-\frac{h}{1500}\right) + A \exp\left(-\frac{h}{100}\right) \tag{2}$$

Herein, *h* represents the height of atmosphere (m), *v* represents the wind velocity in upper layer of atmosphere (m/s), *A* is the scaling factor which represents the fluctuation of the earth's surface (m-2/3). In H-V day model, the scaling factors are *v* = 21 m/s and *A* = 1.7×10-14 m-2/3. This is equivalent to looking up the sky from the earth's surface, and the coherence length of the fluctuation of the atmosphere is 0 *r* = 5 cm. We firstly consider the averaged influence of fluctuation which also means the extensity of beam and degradation of the strength of the central part. We use H-V model to represent the fluctuation of the refractive index of the atmosphere. And we calculate the averaged intensity of light of the receiving point when the Gaussian beam is transmitted from the earth to the sky (uplink). If the atmosphere is not disturbed and it is possible to apply Rytov approximation to the wave equation, the averaged strength of the Gaussian beam after propagating the distance L is :

$$
\langle I(r,L) \rangle = \frac{W\_0^2}{W\_e^2} \exp\left(-\frac{2r^2}{W\_e^2}\right) \tag{3}
$$

Therefore, the anticipated value of the received beam profile still remains Gaussian distribution, and the variant is given by the effective beam radius as follows:

$$\mathcal{W}\_c = \mathcal{W} \left( \mathbf{1} + \mathbf{G}\_{\mu} \right)^{\mathbf{K}} \tag{4}$$

And,

$$\mathbf{G}\_{\mu} = \mathbf{4.35} \mu\_1 \Lambda^{\gtrless\lambda} k^{\gtrless\lambda} \left( H - h\_0 \right)^{\gtrless\lambda} \sec^{\prime\prime} \xi \left( \xi \right) \tag{5}$$

$$\mu\_1 = \int\_{h\_0}^{\cdot} \mathbb{C}\_n^2 \left( h \right) \xi\_1^{\xi\_1^{\prime}} dh, \ \xi\_1 = 1 - \frac{h - h\_0}{H - h\_0} \tag{6}$$

are the parameters which represents the influence of fluctuation, *W*0 is the radius of transmitting beam, κ is the wave number, *L* is the distance between the transmitting system and the receiving system, 0 *h* is the height between the earth's surface and the transmitting system, ξ is zenith angle and ( ) <sup>0</sup> *Hh L* = + cos ξ is the height of the receiving system in the sky. Also, *W* is the radius without the fluctuation of the atmosphere, and it is possible to describe *W* using 0*F* (the curvature radius of exit point of the transmitting light) as follows:

$$\mathcal{W} = \mathcal{W}\_0 \sqrt{\left(1 - \frac{L}{F\_0}\right)^2 + \left(\frac{2L}{k\mathcal{W}\_0^2}\right)^2} = \mathcal{W}\_0 \sqrt{\Theta\_0^2 + \Lambda\_0^2} \tag{7}$$

Herein:

204 Optical Communication

κ

0

And,

transmitting beam,

κ

Herein, 00 0 5.92 / , 2 / *<sup>i</sup>*

 κπ= = *l L*

( ) ( )

ν

length of the fluctuation of the atmosphere is 0

and the size of the fluctuation of the atmosphere are used:

0 <sup>11</sup> <sup>2</sup> 22 2 <sup>6</sup>

*i*

(1)

κ

 

<sup>−</sup>

(2)

*r* = 5 cm. We firstly consider the averaged

<sup>2</sup> 1 *WW G e u* = + (4)

ξ

<sup>−</sup> <sup>=</sup> = − <sup>−</sup> (6)

0

*H h*

is the wave number, *L* is the distance between the transmitting

(3)

(5)

<sup>2</sup> ( ) 0.033 ( ) exp *n n*

Φ= + −

*<sup>K</sup> K CK* κ

*l* is the inner scale, 0 *L* is the outer scale. Based on the actual measurements of the change of <sup>2</sup> *Cn* (refractive index structure constant) in high altitude, Hufnagel-Valley model (H-V5/7 model) parameterized by the scaling factors of wind speed in upper layer of the atmosphere

<sup>2</sup> <sup>10</sup> <sup>2</sup> <sup>5</sup> <sup>16</sup> 0.00594 10 exp 2.7 10 exp exp <sup>27</sup> <sup>1000</sup> <sup>1500</sup> <sup>100</sup> *<sup>n</sup> h hh C h <sup>h</sup> <sup>A</sup>*

Herein, *h* represents the height of atmosphere (m), *v* represents the wind velocity in upper layer of atmosphere (m/s), *A* is the scaling factor which represents the fluctuation of the earth's surface (m-2/3). In H-V day model, the scaling factors are *v* = 21 m/s and *A* = 1.7×10-14 m-2/3. This is equivalent to looking up the sky from the earth's surface, and the coherence

influence of fluctuation which also means the extensity of beam and degradation of the strength of the central part. We use H-V model to represent the fluctuation of the refractive index of the atmosphere. And we calculate the averaged intensity of light of the receiving point when the Gaussian beam is transmitted from the earth to the sky (uplink). If the atmosphere is not disturbed and it is possible to apply Rytov approximation to the wave equation, the averaged strength of the Gaussian beam after propagating the distance L is :

0

= −

Therefore, the anticipated value of the received beam profile still remains Gaussian

*<sup>W</sup> <sup>r</sup> IrL*

, exp

1 0 4.35 sec *G k Hh <sup>u</sup>* =Λ −

( ) <sup>5</sup> 3

 ξξ

1 11

2 2

 

2 2 2

*e e*

*W W*

( ) <sup>1</sup>

2 0

*h h C h dh*

are the parameters which represents the influence of fluctuation, *W*0 is the radius of

system and the receiving system, 0 *h* is the height between the earth's surface and the

, 1

( ) ( ) <sup>5</sup> <sup>5</sup> <sup>7</sup> <sup>11</sup> <sup>6</sup> 6 6 <sup>6</sup>

( )

distribution, and the variant is given by the effective beam radius as follows:

μ

0

*H n h*

μ

− − <sup>=</sup> − +× − + −

$$\Lambda = \frac{2L}{k\mathcal{W}^2} = \frac{\Lambda\_0}{\Theta\_0^2 + \Lambda\_0^2}, \quad \Lambda\_0 = \frac{2L}{k\mathcal{W}\_0^2} \tag{8}$$

$$\Theta = 1 + \frac{L}{F} = \frac{\Theta\_0}{\Theta\_0^2 + \Lambda\_0^2}, \ \Theta\_0 = 1 - \frac{L}{F\_0} \tag{9}$$

$$\mathcal{W}\_{\boldsymbol{c}} = \mathcal{W} \left( \mathbf{1} + \mathbf{G}\_{\boldsymbol{d}} \right)^{\underline{\mathbf{Y}}} \tag{10}$$

On the other hand, regarding the downlink, the following values are substituted into the equation (3).

$$\mathbf{G}\_d = 4.35 \mu\_2 \Lambda^s k^s \mathbf{\tilde{X}}^s \left(\mathbf{H} - \mathbf{h}\_0\right) \mathbf{\tilde{X}}^s \mathbf{\tilde{X}}^s \left(\mathbf{\tilde{\xi}}\right) \tag{11}$$

$$\mu\_2 = \int\_{h\_0}^H \mathbb{C}\_n^2(h) \, \xi\_0^{\xi\_0^\prime} dh, \, \xi\_0 = \frac{h - h\_0}{H - h\_0} \tag{12}$$

With the above-mentioned formulas, we will discuss in the next section with the demand of optical specifications of the optical antenna available for NG-FSO system and RoFSO system.

#### **4. Fine tracking technology**

The challenge in all-optical connection of FSO and SMF systems is not only to design an effective beam tracking and optical antenna alignment technique, but also an efficient method for focusing the light into the SMF at the receiver. Active tracking is required to maintain alignment of the received optical signal to the SMF. To achieve stable beam spot position control as well as compensate for beam AOA fluctuation caused by atmospheric turbulence we adopted a small size two axis galvanometer type mirror drive mechanism. This mirror device called fine pointing mirror (FPM) is shown in figure 2. The tracking system constitutes the feedback system which considers the direction of the optic axis of a received optical system which changes the arrival directions of beacon light with an input and inclination of FPM as an output. Figure 3 shows a block diagram of a tracking servo system. Although the actual servo system consists of two horizontal independent axes this

**Figure 2.** Photograph of FPM

**Figure 3.** Block diagram of the fine tracking servo system

**Figure 4.** Frequency characteristics of closed loop feedback system. (a) Amplitude (b) Phase

figure shows only one axis. The position of the received beam is determined by calculation the position of the beacon spot on the four elements of the QD. This signal serves as an input of an analog proportional–integral–derivative (PID) controller. The transfer characteristic of FPM is carrying out the response of a typical secondary system (azimuth (Az) axis is 100 Hz and elevation (El) axis of resonance frequency is 80 Hz at the weak oscillating system of dumping), and the PID controller by progress and delay compensation is designed so that a control zone may become the largest in consideration of this characteristic and the nonlinearity of a tracking sensor. As a result, the control zone of the closed loop response has attained the performance of about 2 kHz. In the case of a feedback system, disturbance suppression performance is proportional to a closed loop zone. Therefore, this tracking system shows that tracking control more highly efficient a single figure than the conventional tracking system which was an about 100 Hz control zone was realizable. Disturbance is poured in from the integration input of Figure 4, and the result of having measured the tracking response which appears in a mirror drive output about Az axis is shown in Figure 5. A vertical axis is the amount of suppression of disturbance amplitude. The characteristic is the same also about El axis. On the frequency of 100 Hz or less, it turns out that it has the capability that disturbance can be suppressed or less to 1/100.

**Figure 5.** Turbulence suppression characteristics of feedback system **Frequency [Hz]**

### **5. Research and development on NG-FSO system**

#### **5.1. The basic concept of the NG-FSO**

206 Optical Communication

**Figure 2.** Photograph of FPM

+ -


**Amplitude (dB)**

Angle of arrival

**Figure 3.** Block diagram of the fine tracking servo system

10 100 1000 10000 **Frequency (Hz)**

*Mirror angle*

Tracking sensor (QD)

**Figure 4.** Frequency characteristics of closed loop feedback system. (a) Amplitude (b) Phase

Differentiation compensation

**PID controller**

Integration compensation Fine pointing mirror

10 100 1000 10000 **Frequency (Hz)**

Pointing angle

*FPM*

out that it has the capability that disturbance can be suppressed or less to 1/100.

figure shows only one axis. The position of the received beam is determined by calculation the position of the beacon spot on the four elements of the QD. This signal serves as an input of an analog proportional–integral–derivative (PID) controller. The transfer characteristic of FPM is carrying out the response of a typical secondary system (azimuth (Az) axis is 100 Hz and elevation (El) axis of resonance frequency is 80 Hz at the weak oscillating system of dumping), and the PID controller by progress and delay compensation is designed so that a control zone may become the largest in consideration of this characteristic and the nonlinearity of a tracking sensor. As a result, the control zone of the closed loop response has attained the performance of about 2 kHz. In the case of a feedback system, disturbance suppression performance is proportional to a closed loop zone. Therefore, this tracking system shows that tracking control more highly efficient a single figure than the conventional tracking system which was an about 100 Hz control zone was realizable. Disturbance is poured in from the integration input of Figure 4, and the result of having measured the tracking response which appears in a mirror drive output about Az axis is shown in Figure 5. A vertical axis is the amount of suppression of disturbance amplitude. The characteristic is the same also about El axis. On the frequency of 100 Hz or less, it turns

(a) Amplitude (b) Phase


**Phase (deg.)**

In research and development of the NG-FSO system, we developed a compact optical antenna suitable for optical communication between satellite in space and deployment in stratospheric platform [Arimoto 2003][Katsuo 2005]. Because of the limitation in payload and equipment size in such usage, a lightweight optical antenna was required designed using off-axis free-form optics without the obscuration [Takahashi 2006]. In general, a Galileo type optical system, composed of a convex lens and a concave lens, is used to convert input/output diameter of laser beam in an optical antenna. However, when using such kind of refractive optical system it is difficult to suppress reflection of each surface of lenses in a wide wavelength range, so measurement errors as a result of ghost and/or flare can arise. In addition, since this system is accompanied with a chromatic aberration, adjustment of the focal length based on a wavelength is necessary. Therefore, a reflection based optical system which does not have a chromatic aberration is suitable for an optical system that is incorporated into a highly precise optical antenna. Although there are many cases that a Cassegrain type configuration is used for beam expander of reflecting mirror system, it is not suitable for the optics of the antenna of this purpose because of an obscuration in the beam center caused by secondary mirror. We abandoned the constitution on co-axial optical system and devised off-axial reflection optical system instead. In case of optical wireless communication on ground or between ground and space, the fluctuation of arrival beam angle is caused by the atmospheric turbulence. Therefore, if comatic aberration of the optical antenna is large, beam expander causes peculiar aberrations according to an incident angle of laser beam. Because rotational asymmetric aberration (shown in Figure 6(b)) by the decentering of the optical element occurs as well as radial aberration such as the comatic aberration, in the case of an off-axial optical system, all those aberration becomes able to be compensated by using free-form surface for all mirrors. The free-form surface [Takahashi 2011] is curved surface defined in XY polynomial such as equation 13.

$$Z = \sum\_{n=0}^{k} \sum\_{m=0}^{n} \mathbb{C}\_{nm} X^{m} Y^{n-m} \tag{13}$$

In one aspect, free-form surface becomes able to have positive and negative power (Figure 6(a)). Furthermore, the curvature of the tangential direction can be different from azimuth direction.

**Figure 6.** A free-form surface and Asymmetrical aberrations

### **5.2. Consideration of the optical specifications**

The average expansion and fluctuation of light intensity of the laser beam at 1.550 μm wavelength with the atmospheric turbulence are calculated by using the expressions of Section 3 stated above. In this calculation, the transmission beam is Gaussian beam which have beam divergence of diffractive limit, the difference in altitude between the two stations (low altitude stratospheric platform experiment [Katsuo 2005] condition) is 4 km, zenith angle is 60 degree, the transmission distance of the beam is 8 km maximum, and the pointing errors is 0 μradian. When the transmission beam radius is 40 mm, the free space loss is approximately 1 dB. So that if the optical antenna achieves an output transmission of around 100 mW and the receiver sensitivity of -20 dB, we can expect a margin of 40 dB and it is possible to communicate under thin cloud or fog conditions. The relation between the beam radius at the transmitter (W0) and the average beam expansion at receiver point is shown in Figure 7. The mean intensity fluctuation of the received beam under similar conditions is shown in Figure 8. According to these results, the radius of received beam is varied slightly and the expansion rate of the radius of received beam is constant when the radius of transmit beam is more than 60 mm. Moreover, the larger the transmission beam diameter is, the smaller the amount of the decrease of center strength of the received beam, and the free space transmission loss is less than 5 dB for a transmit beam diameter of uplink 70 mm or downlink 50 mm or more. Assuming Cn2 is 1.7×10-14 (H/V model), we calculated the fluctuation of the received light intensity (scintillation index) by varying the transmitted beam radius. The result is shown in figure 9.

The change is small in weak fluctuation case and there is no big received optical power changes for propagation distance of ~4 km with laser beam transmit and receive aperture diameter of 10~100 mm therefore an optical antenna having this range of aperture beam transmission is possible. Because this fluctuation is significantly small and in the region of weak fluctuation and there is not large fluctuation of intensity of the received beam, the optical antenna which has such diameter of aperture transmits the optical beam have the possibility. The calculation result of receive light intensity fluctuation when the transmit beam radius is set at 20 mm and pointing error is 1 μradian are shown in figure 10. On the assumption that the inoperable rate is 10-7, it is necessary to consider the fading margins which are 3.6 dB for the uplink and 5.3 dB for the downlink. From these calculations, it is desirable for the pointing error to be around 1 μradian in consideration of fading when a diffraction limited Gaussian beam is transmitted.

The pointing error of the FPM used for the fine tracking of the NG-FSO system in this research is estimated to be approximately 20 μradian. The angular magnification of the optical antenna is 20, and the tracking accuracy in the system can expect the improvement of the atmosphere fluctuation to become 1.0 μradian in a calculation. Considering the above outlined analytical results, the target specifications of the optical system are as follows:


208 Optical Communication

direction.

0 0

*Z C XY* <sup>−</sup> = =

In one aspect, free-form surface becomes able to have positive and negative power (Figure 6(a)). Furthermore, the curvature of the tangential direction can be different from azimuth

The average expansion and fluctuation of light intensity of the laser beam at 1.550 μm wavelength with the atmospheric turbulence are calculated by using the expressions of Section 3 stated above. In this calculation, the transmission beam is Gaussian beam which have beam divergence of diffractive limit, the difference in altitude between the two stations (low altitude stratospheric platform experiment [Katsuo 2005] condition) is 4 km, zenith angle is 60 degree, the transmission distance of the beam is 8 km maximum, and the pointing errors is 0 μradian. When the transmission beam radius is 40 mm, the free space loss is approximately 1 dB. So that if the optical antenna achieves an output transmission of around 100 mW and the receiver sensitivity of -20 dB, we can expect a margin of 40 dB and it is possible to communicate under thin cloud or fog conditions. The relation between the beam radius at the transmitter (W0) and the average beam expansion at receiver point is shown in Figure 7. The mean intensity fluctuation of the received beam under similar conditions is shown in Figure 8. According to these results, the radius of received beam is varied slightly and the expansion rate of the radius of received beam is constant when the radius of transmit beam is more than 60 mm. Moreover, the larger the transmission beam diameter is, the smaller the amount of the decrease of center strength of the received beam, and the free space transmission loss is less than 5 dB for a transmit beam diameter of uplink 70 mm or downlink 50 mm or more. Assuming Cn2 is 1.7×10-14 (H/V model), we calculated the fluctuation of the received light intensity (scintillation index) by varying the transmitted

The change is small in weak fluctuation case and there is no big received optical power changes for propagation distance of ~4 km with laser beam transmit and receive aperture diameter of 10~100 mm therefore an optical antenna having this range of aperture beam

*n m*

**Figure 6.** A free-form surface and Asymmetrical aberrations

x

(a) Example of a free-form surface

z y

**5.2. Consideration of the optical specifications** 

beam radius. The result is shown in figure 9.

*k n m nm nm*

y

<sup>=</sup> (13)

z *Trapezoid Tangential*

(b) Asymmetrical aberrations

Axial comatic aberration Asymmetrical Distortion


**Figure 7.** Rx beam radius versus Tx beam radius

**Figure 8.** Mean intensity depend on Tx beam radius

**Figure 9.** Scintillation index depend on Tx beam radius

**Figure 10.** Probability of fading level

#### **5.3. Result of optical design**

In order to conduct an optical system without obscuration, having less deterioration of wavefront accuracy when the incident beam angle varies, we designed an optimum optical system comprising off-axial free form surface triple mirror which have enlargement/reduction rate of 20 times. Optical layout of triple mirror is shown in figure 7. Fundamental power placement in an optical design is a primary concave mirror plus a secondary convex mirror. With this optical configuration, a primary image is formed whose spherical and coma aberrations are compensated, and by using a collimator mirror which is arranged after the primary image a beam of parallel rays of light is producing. We set the paraxial focal length of the primary mirror and the secondary mirror at f1=230 mm, the collimator mirror at f2=11.8 mm, and the angular magnification of the triple mirror at approximately 20. Moreover, we have decentered aberrations, such as axial comatic aberration and axial astigmatism caused by tilt and decentered of both reflective surfaces, compensated by the three pieces of free-form mirror. A figure of constitution of the prototype of the optical antenna module which had a QD feedback type built-in fine tracking system by a microminiaturized fine pointing mirror (FPM) is shown in figure 11, the photograph is shown in figure 12, and the photograph is shown in figure 13. Based on the design result, the calculations of coupling efficiency when an ideal lens is deployed in the radiation side are shown in figure 14.

**Figure 11.** Optical layout of an optical antenna with off-axial free form optics.

**Figure 9.** Scintillation index depend on Tx beam radius

Feding level (dB)

0 0.02 0.04 0.06 0.08 0.1 0.12

Scintillation index

In order to conduct an optical system without obscuration, having less deterioration of wavefront accuracy when the incident beam angle varies, we designed an optimum optical system comprising off-axial free form surface triple mirror which have enlargement/reduction rate of 20 times. Optical layout of triple mirror is shown in figure 7. Fundamental power placement in an optical design is a primary concave mirror plus a secondary convex mirror. With this optical configuration, a primary image is formed whose spherical and coma aberrations are compensated, and by using a collimator mirror which is arranged after the primary image a beam of parallel rays of light is producing. We set the paraxial focal length of the primary mirror and the secondary mirror at f1=230 mm, the collimator mirror at f2=11.8 mm, and the angular magnification of the triple mirror at approximately 20. Moreover, we have decentered aberrations, such as axial comatic aberration and axial astigmatism caused by tilt and decentered of both reflective surfaces, compensated by the three pieces of free-form mirror. A figure of constitution of the prototype of the optical antenna module which had a QD feedback type built-in fine tracking system by a microminiaturized fine pointing mirror (FPM) is shown in figure 11, the photograph is shown in figure 12, and the photograph is shown in figure 13. Based on the design result, the calculations of coupling efficiency when an

1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 Probability

0 0.01 0.02 0.03 0.04 0.05 0.06

UPLINK W0 DOWNLINK W0

Tx beam radius (m)

UPLINK DOWNLINK

ideal lens is deployed in the radiation side are shown in figure 14.

**Figure 10.** Probability of fading level

**5.3. Result of optical design** 

**Figure 12.** Appearance of prototype of optical antenna module

**Figure 13.** Photograph of prototype optical antenna on the gimbal stage

**Figure 14.** Coupling efficiency depend on internal field of view

#### **5.4. Experimental set up of NG-FSO system**

The test beds were put on two campuses of the Waseda University campus area situated in Shinjuku Ward, Tokyo city [Kazaura 2007][Kazaura 2006]. Figure 11 shows a map view of where the FSO systems are deployed. Two antennas were set on the rooftop of a 10 floor building (figure 15) and the corresponding antennas on the rooftop of a 9 floor building with a link spanning 1 km between the transceivers on the top of those two buildings. The antenna shown on the right in figure 16 is the antenna under experiment while the antenna on the left is used for scintillation and optical power attenuation measurement. They are both connected to data acquisition system placed in the experiment setup room which records all the data as shown in figure 17. The FSO beam propagation path travels over low rising residential, office or campus buildings, thus transmitted beam will experience higher probability of scintillation from ground heating and heating from these buildings. The experimental antennas use a 980 nm beacon for alignment and 1550 nm wavelength for communication. A charge coupled device (CCD) camera is used for initial alignment purposes which is done manually adjusting thimble screws located at the base of the antenna. Repeated adjustments are made on both sides until the light is centered on both CCD cameras (see TV monitor in figure16). Fine alignment of the antennas is achieved by using a QD which gets a feedback of the maximum received power from the inbuilt data acquisition system. The 1550 nm transmission beam was driven by an erbium doped fiber amplifier (EDFA) capable of outputting 100 mW total power. The experimental antenna can be controlled from a remote location by a serial computer interface to send control information that is used for remote alignment. For the scintillation and optical power attenuation measurement antennas, a serial computer interface provides the beam output power and receives signal strength information and other information.

The basic configuration of our experimental setup for the developed FSO system by alloptical connection of free-space and SMF is show in Figure 18. Because the signal light is fiber coupled at both ends, booster/post EDFA and other measurement and data collection devices can be conveniently placed inside the building. The fibers are run to the respective rooftops and then coupled directly to the antenna as shown in Figure 16. Separation of transmitting light and receiving light is performed by an optical circulator. The test signal Next Generation Optical Wireless Communication Systems Using Fiber Direct Coupled Optical Antennas 213

212 Optical Communication

**Figure 14.** Coupling efficiency depend on internal field of view

power and receives signal strength information and other information.

The basic configuration of our experimental setup for the developed FSO system by alloptical connection of free-space and SMF is show in Figure 18. Because the signal light is fiber coupled at both ends, booster/post EDFA and other measurement and data collection devices can be conveniently placed inside the building. The fibers are run to the respective rooftops and then coupled directly to the antenna as shown in Figure 16. Separation of transmitting light and receiving light is performed by an optical circulator. The test signal

The test beds were put on two campuses of the Waseda University campus area situated in Shinjuku Ward, Tokyo city [Kazaura 2007][Kazaura 2006]. Figure 11 shows a map view of where the FSO systems are deployed. Two antennas were set on the rooftop of a 10 floor building (figure 15) and the corresponding antennas on the rooftop of a 9 floor building with a link spanning 1 km between the transceivers on the top of those two buildings. The antenna shown on the right in figure 16 is the antenna under experiment while the antenna on the left is used for scintillation and optical power attenuation measurement. They are both connected to data acquisition system placed in the experiment setup room which records all the data as shown in figure 17. The FSO beam propagation path travels over low rising residential, office or campus buildings, thus transmitted beam will experience higher probability of scintillation from ground heating and heating from these buildings. The experimental antennas use a 980 nm beacon for alignment and 1550 nm wavelength for communication. A charge coupled device (CCD) camera is used for initial alignment purposes which is done manually adjusting thimble screws located at the base of the antenna. Repeated adjustments are made on both sides until the light is centered on both CCD cameras (see TV monitor in figure16). Fine alignment of the antennas is achieved by using a QD which gets a feedback of the maximum received power from the inbuilt data acquisition system. The 1550 nm transmission beam was driven by an erbium doped fiber amplifier (EDFA) capable of outputting 100 mW total power. The experimental antenna can be controlled from a remote location by a serial computer interface to send control information that is used for remote alignment. For the scintillation and optical power attenuation measurement antennas, a serial computer interface provides the beam output


Internal field of view (μrad)

EL AZ

**5.4. Experimental set up of NG-FSO system** 

coupling efficiency(dB)

**Figure 15.** A map view of the 1km test between Waseda University Nishi-Waseda campus and Okubo campus

**Figure 16.** Optical antennas on the rooftop of building 14 in Nishi-Waseda

(pseudo random bit sequence) from bit error rate tester (BERT) is changed into a light signal by an E/O converter, and after being amplified with a booster EDFA, it is sent to the optical antenna of the rooftop. At the other end after the optical signal received with the optical antenna is removed in amplification by post-EDFA and a light filter removes noise, it is changed into an electric signal by an O/E converter, and becomes a receiving bit sequence in a clock data recovery circuit, and calculation of the bit error is carried out by BERT. The light signal which branched by 3 dB-coupler on the way is used for the monitor of receiving intensity, or other measurement. In a WDM experiment, the wavelength of each signal is made to fit the ITU grid (100 GHz channel spacing), and multi/de-multiplex is performed using a DWDM multi/de-multiplex device. Moreover, a weather monitor device and another optical antenna are installed in the rooftop and weather condition (visibility, precipitation and temperature) and link line condition (scintillation and optical power attenuation) are measured simultaneously.

**Figure 17.** Photograph of experimental hardware setup

**Figure 18.** Experimental FSO communication system setup
