**2.2 Classification of FOG**

The development of FOG can be roughly divided into three generations: interferometric FOG, resonant FOG, and stimulated Brillouin scattering FOG [11], as shown in **Table 2**.

### **2.3 Basic composition of FOG**

FOG is based on solid-state technology of optical fiber communication. Specifically, the main components of FOG are shown in **Figure 2** [15]:


Note that with proper design and components, FOG performance is repeatable in production, even for high-performance terminals.

### **2.4 Principle of FOG**

#### *2.4.1 Interferometric FOG*

When the whole system rotates, two beams of light propagating in the opposite direction produce phase difference, and the interference intensity changes. Interferometric FOG can calculate the rotation angular velocity according to the intensity change detected by the optical detector.

The light emitted by the light source is divided into two identical beams through the beam splitter, which propagate in a closed optical path counterclockwise and

**27**

**Table 2.**

*The classification of FOG.*

*On the Development and Application of FOG DOI: http://dx.doi.org/10.5772/intechopen.88542*

Index Zero-biased stability: (°/h):

Main features The SAGNAC effect is

complex

DepolarizedI-FOG ALLPM-fiberI-FOG IOCI-FOG

Aircraft and vehicle navigation, missile guidance, precision space vehicle, submarine [11]

life, high reliability, no mechanical vibration, anti-electromagnetic interference, light weight, small size, high sensitivity, wide bandwidth, easy to realize multi-channel or distributed sensors [12]

affected by temperature. As the length increases, the cost becomes more and more expensive, the accuracy cannot be improved, and the miniaturization cannot be

Advantages Low random walk, long

Shortcomings Optical fibers are greatly

achieved [14]

Specific classification

Sample grap

Stage of development

Application area

8.5129 angle random walk coefficient (ARW) is 0.0841°/h~(1/2)

enhanced by using multiturn fiber coils. A doublebeam ring interferometer consisting of multi-turn single-mode fiber coils can provide high accuracy and will inevitably make the overall structure more

**The first generation The second generation The third generation**

Zero-biased stability:

18.181 angle random walk coefficient (ARW) is 0.05781°/h~(1/2)

Ring resonator is used to enhance SAGNAC effect and cycle propagation is used to improve accuracy. Therefore, shorter optical fibers can be used

ALLfiberR-FOG IOCR-FOG(MOG)

laboratory to practice

Compared with I-FOG, the theoretical accuracy is more accurate and the volume is smaller

The production cannot meet the current demand

— —

Practical stage Stage of transition from

scattering FOG

The response of threshold power of pump light to temperature is 32.6 × 10–6°C−<sup>1</sup>

the response of beat frequency to temperature is 88.232.6 × 10–6°C−<sup>1</sup>

Conversion of light power into light wave by stimulated Brillouin

SBS-FOG(B-FOG)

Theoretical stage

Simple structure, few parts, firm and stable, strong shock resistance and acceleration resistance, long service life, high sensitivity and resolution, instantaneous start-up in principle and wide dynamic range [13]

Generation and stable output of singlefrequency SBS laser, locking phenomenon, polarization fluctuation, temperature effect

scattering

, and

Name Interferometric FOG Resonant FOG Stimulated Brillouin

(°/h):


*On the Development and Application of FOG DOI: http://dx.doi.org/10.5772/intechopen.88542*

*Gyroscopes - Principles and Applications*

**2.2 Classification of FOG**

**2.3 Basic composition of FOG**

filtering with fiber Bragg grating.

and kilometers in high-grade).

exchange waveguide.

of the interferometer.

**2.4 Principle of FOG**

*2.4.1 Interferometric FOG*

shown in **Table 2**.

of inertial navigation technology. FOG has great value in the military field, because of its remarkable advantages, flexible structure, and broad application prospects. It has attracted the attention of universities and scientific research institutions in many countries in the world, and has invested a lot of energy in research. At the end of 1980s and the beginning of 1990s, FOG technology has been widely used. Its sensitivity has been improved by four orders of magnitude, and the angular velocity measurement accuracy has been improved from the initial 15°/h to 0.001°/h.

The development of FOG can be roughly divided into three generations: interferometric FOG, resonant FOG, and stimulated Brillouin scattering FOG [11], as

FOG is based on solid-state technology of optical fiber communication.

1.AAA, an advanced broadband source based on EDFA technology, has a wavelength of 1550 nm. Wavelength stability can be obtained by internal spectral

2.Polarization-maintaining optical fiber coils (hundreds of meters in mid-range

3.The integrated optical circuit of Linbo3 with electrodes is used to generate phase modulation and provide good polarization selectivity through proton

4.An optical fiber coupler (or circulator for higher return power) for transmitting signals to the detector light returned from the common input-output port

6.The digital logic electronic device that generates phase modulation and phase

Note that with proper design and components, FOG performance is repeatable

When the whole system rotates, two beams of light propagating in the opposite direction produce phase difference, and the interference intensity changes. Interferometric FOG can calculate the rotation angular velocity according to the

The light emitted by the light source is divided into two identical beams through the beam splitter, which propagate in a closed optical path counterclockwise and

5.Analog-to-digital (A/D) converter for sampling detector signals.

feedback through a digital-to-analog converter.

in production, even for high-performance terminals.

intensity change detected by the optical detector.

Specifically, the main components of FOG are shown in **Figure 2** [15]:

**26**

#### **Table 2.** *The classification of FOG.*

#### **Figure 3.**

*The principle of interferometric FOG.*

clockwise, respectively. The two beams will interfere at the beam splitter. If the closed optical path does not rotate relative to the inertial space, the two beams pass through the same path and the phase difference is zero. If the closed optical path has a rotational angular velocity relative to the inertial space, the two beams experience different paths with a slight optical path difference. At the same time, the two beams also have a phase difference, which is the Sagnac effect. IFOG uses Sagnac effect to measure rotation angular velocity. The interferometric FOG is actually the Sagnac interferometer [16]. Its schematic diagram is shown in **Figure 3**.

#### *2.4.2 Resonant FOG*

The basic principle of resonant FOG is Sagnac effect. The core device of resonant FOG is fiber ring resonator. The limit sensitivity of resonant FOG is determined by the shot noise of photodetector, so it is closely related to the resonant characteristics of resonant cavity.

**29**

**Figure 4.**

*The reflective resonator.*

*On the Development and Application of FOG DOI: http://dx.doi.org/10.5772/intechopen.88542*

sured, the rotation rate Ψ can be learned.

*2.4.3 Stimulated Brillouin scattering FOG*

and beating the frequency.

quency point of the light wave in a certain direction.

sion spectrum of the resonator to detect bright peaks [17].

propagates in the optical fiber loop with periodic interference. The light source with narrow linewidth has the characteristics of long coherence, which results in resonance effect. The change of optical path of light wave propagating in optical fiber loop will lead to the change of resonance frequency point. The corresponding angular velocity can be obtained by obtaining the change of the resonance fre-

RFOG is divided into two types, including reflective and transmission ring resonators. As shown in **Figure 4**, the reflective type uses the reflection spectrum of the resonator to detect dark peaks, while the transmission type uses the transmis-

The basic principle of RFOG is Sagnac effect. For resonant gyroscope, its output detects the frequency difference of clockwise and counterclockwise beams propagating in the resonant cavity. Because it is sensitive to Sagnac frequency shift by using the steep resonant curve of the resonant cavity, it greatly reduces the length of the sensitive fiber optic coil. When the resonant cavity is stationary, the frequency difference between the two beams is zero. When the resonator rotates, the frequency of two beams of light propagating in opposite directions changes, and the frequency difference of the two beams is linear with the rotational speed. It is this frequency difference signal that the resonator gyroscope detects. Its expression is [18, 19] *Δv* = *vccw* <sup>−</sup>*vcw* = \_

4*A <sup>L</sup> <sup>Ω</sup>* = \_ *D*

where A is the area of the resonator, D is the diameter of the resonator, and 4A/λL or D/λ is the scale factor of the gyroscope. Therefore, as long as Δν is mea-

Stimulated Brillouin scattering occurs when the intensity of the transmitted light in the fiber ring reaches threshold level. The frequency of the scattered light varies with the rotation angular velocity of the fiber ring due to the influence of Sagnac effect. The rotation angular velocity of the optical fiber ring can be obtained by detecting the frequency of the scattered light produced by CW and CCW light

Stimulated Brillouin FOG is a gyroscope consisting of Brillouin laser. It is an optical product of Ring Laser Gyroscope (RLG). Its basic principle is shown in **Figure 5**. When the incident light intensity exceeds the Brillouin threshold of the optical fiber, due to the electrostrictive effect, a moving acoustic wave will be generated

<sup>λ</sup> *<sup>Ω</sup>* (1)

Resonant Fiber Optic Gyroscope (RFOG) is also based on the clockwise and counterclockwise optical path changes caused by Sagnac effect. The light wave

#### *On the Development and Application of FOG DOI: http://dx.doi.org/10.5772/intechopen.88542*

*Gyroscopes - Principles and Applications*

**Figure 2.**

**Figure 3.**

*The basic composition of FOG.*

clockwise, respectively. The two beams will interfere at the beam splitter. If the closed optical path does not rotate relative to the inertial space, the two beams pass through the same path and the phase difference is zero. If the closed optical path has a rotational angular velocity relative to the inertial space, the two beams experience different paths with a slight optical path difference. At the same time, the two beams also have a phase difference, which is the Sagnac effect. IFOG uses Sagnac effect to measure rotation angular velocity. The interferometric FOG is actually the

The basic principle of resonant FOG is Sagnac effect. The core device of resonant FOG is fiber ring resonator. The limit sensitivity of resonant FOG is determined by the shot noise of photodetector, so it is closely related to the resonant characteristics

Resonant Fiber Optic Gyroscope (RFOG) is also based on the clockwise and counterclockwise optical path changes caused by Sagnac effect. The light wave

Sagnac interferometer [16]. Its schematic diagram is shown in **Figure 3**.

**28**

*2.4.2 Resonant FOG*

*The principle of interferometric FOG.*

of resonant cavity.

propagates in the optical fiber loop with periodic interference. The light source with narrow linewidth has the characteristics of long coherence, which results in resonance effect. The change of optical path of light wave propagating in optical fiber loop will lead to the change of resonance frequency point. The corresponding angular velocity can be obtained by obtaining the change of the resonance frequency point of the light wave in a certain direction.

RFOG is divided into two types, including reflective and transmission ring resonators. As shown in **Figure 4**, the reflective type uses the reflection spectrum of the resonator to detect dark peaks, while the transmission type uses the transmission spectrum of the resonator to detect bright peaks [17].

The basic principle of RFOG is Sagnac effect. For resonant gyroscope, its output detects the frequency difference of clockwise and counterclockwise beams propagating in the resonant cavity. Because it is sensitive to Sagnac frequency shift by using the steep resonant curve of the resonant cavity, it greatly reduces the length of the sensitive fiber optic coil. When the resonant cavity is stationary, the frequency difference between the two beams is zero. When the resonator rotates, the frequency of two beams of light propagating in opposite directions changes, and the frequency difference of the two beams is linear with the rotational speed. It is this frequency difference signal that the resonator gyroscope detects. Its expression is [18, 19]

$$
\Delta \Delta\_v = \mathcal{v}\_{cww} - \mathcal{v}\_{cw} = \frac{4A}{\lambda L} \Delta \mathbf{\hat{Q}} = \frac{D}{\lambda} \Delta \mathbf{\hat{Q}} \tag{1}
$$

where A is the area of the resonator, D is the diameter of the resonator, and 4A/λL or D/λ is the scale factor of the gyroscope. Therefore, as long as Δν is measured, the rotation rate Ψ can be learned.

#### *2.4.3 Stimulated Brillouin scattering FOG*

Stimulated Brillouin scattering occurs when the intensity of the transmitted light in the fiber ring reaches threshold level. The frequency of the scattered light varies with the rotation angular velocity of the fiber ring due to the influence of Sagnac effect. The rotation angular velocity of the optical fiber ring can be obtained by detecting the frequency of the scattered light produced by CW and CCW light and beating the frequency.

Stimulated Brillouin FOG is a gyroscope consisting of Brillouin laser. It is an optical product of Ring Laser Gyroscope (RLG). Its basic principle is shown in **Figure 5**. When the incident light intensity exceeds the Brillouin threshold of the optical fiber, due to the electrostrictive effect, a moving acoustic wave will be generated

**Figure 4.** *The reflective resonator.*

**Figure 5.**

*The principle diagram of stimulated Brillouin FOG.*

in the optical fiber. The existence of this moving acoustic wave leads to the generation of stimulated Brillouin scattering (SBS). When two pumped beams (P1 and P2) are incident into the ring resonator in the opposite direction at the same time, two Brillouin beams (B1 and B2) opposite to the pumped beams will be generated. If the ring resonator is stationary, the two Brillouin beams are proportional to the frequency difference, Dn. Two Brillouin beams are photosynthesized and beat frequency is generated. The rotation rate of the optical fiber resonator can be obtained by measuring the beat frequency, Dn.

The rotation angular velocity of the optical fiber coil is linearly related to the frequency difference of the output two Brillouin beams. The ratio factor is \_ 4*S* <sup>λ</sup> · *nL*, where <sup>λ</sup> is the wavelength of the pumped light, L is the length of the optical fiber coil, *n* is the refractive index of the optical fiber coil, and the area *S* is the area surrounded by the optical fiber coil.

## **2.5 Characteristics of FOG**


**31**

**Table 3.**

*On the Development and Application of FOG DOI: http://dx.doi.org/10.5772/intechopen.88542*

**2.7 State of the art of FOG**

tion of FOG.

Angular random walk coefficient (noise measurement conditions)

Scaling temperature compensation

Environmental adaptability

Improving sensitivity and accuracy of detection

*The key technological breakthroughs of FOG.*

**2.6 Key technological breakthroughs of FOG**

tion showed in **Table 3** to obtain better performance for FOG.

many types of FOG have been put into use in the United States.

Back Rayleigh scattering in optical fibers and back scattering from optical interfaces

Important devices are sensitive to temperature

Vibration, shock, acceleration, etc.

Poor performance of functional components

Although FOG has many advantages over other gyroscopes, it still has some shortcomings because of the imperfect technology. Thus, we can employ some solu-

FOG has different development and research status in different countries and has its own characteristics. The United States, Japan, France, Germany, Britain, and China are the main developing countries of FOG. Europe and the United States have obvious advantages in the research and development of high-precision FOG, while Japan pays more attention to the commercial application of low-precision FOG [13]. China and other countries also attach great importance to the research and promo-

The United States is a pioneer in developing and applying FOG. Its contractors, universities, and government agencies are developing key technologies, such as Litton, Honeywell, KVH, Norhrop Grumman, and Draper Laboratory. These companies are mainly engaged in the research and development of high-precision FOG [23], providing services for the U.S. military and aerospace departments. They have also done very well in the development and production of FOG. At present,

Japan is also a big country in the research and production of FOG. The research

Relative intensity noise of light source, thermal phase noise of optical fiber coil and photodetector noise

Average wavelength of

Poor environmental adaptability

Low sensitivity and

accuracy

light source

Noise filtering technology; noise elimination technology

Broadband Erbium doped fiber light source (SFS) [21] with better wavelength stability or wavelength control

feedback control loop is added based on the closed-loop feedback control circuit of FOG by using error signal [22]

Expanding the dynamic range of measuring rotation velocity

Improving matching and phase shift of functional components [12]

[20].

measures

Feedback channel gain The second closed-loop

institutes include the cutting-edge technology laboratory of Tokyo University, Hitachi Corporation, Mitsubishi Corporation [13], Japan Aerospace Electronics Company (JAE), Mitsubishi Precision Instrument, and so on. These companies

**Technical direction Causes Influence factor Solution**

