**3.1. Blue GaN external-cavity diode laser system**

In this subsection, the results of blue ECDL system are presented [15]. Two bulk diffraction gratings are used in the blue ECDL system, one is a holographic diffraction grating with a groove density of 2400 lines/mm (Thorlabs, GH13-24 U), and the zeroth- and first-order diffraction efficiencies are 78.8 and 8.3%. The other is a ruled diffraction grating that is ruled with 1800 lines/mm and has a blaze wavelength of 500 nm (Thorlabs, GR13-1850), the zeroth- and first-order diffraction efficiencies are 29.6 and 53.5%, respectively. The length of the external cavity is around 110 mm.

In the freely running condition, the threshold current of the laser device is around 0.12 A, and the slope efficiency is around 1.62 W/A. With an injected current of 1.4 A, a 2 W output power is obtained. The diode laser is operating in multiple Fabry-Perot (FP) modes. The spectrum is centered around 456 nm with a spectral bandwidth around 1.2 nm. The longitudinal mode spacing of the FP modes is around 28.5 pm.

The blue ECDL system is characterized by measuring the spectrum at different wavelengths with these two different gratings. **Figure 2** shows the spectra of the blue ECDL system with an output power around 85 mW for both gratings. For the ECDL system with the holographic diffraction grating at an injected current of 0.2 A, **Figure 2(a)** shows seven normalized spectra from 453.4 to 456.5 nm. The bandwidth of the spectrum (FWHM) is around 9 pm in the 3.1 nm tunable range; it is much less than the 28.5 pm mode space of the FP laser resonator; this means the laser is forced to operate in a single longitudinal FP mode by the external feedback. **Figure 2(b)** shows eight normalized spectra from 452.8 to 458.8 nm for the ECDL system with the ruled diffraction grating at an injected current of 0.3 A. The spectral bandwidth is less than 20 pm for wavelengths longer than 456 nm in the 6.0 nm tunable range; for shorter wavelengths, the spectral bandwidth is less than 42 pm, that is, two FP modes oscillate simultaneously. The amplified spontaneous emission (ASE) is more than 20 dB suppressed in the tunable ranges for both diffraction gratings.

**Figure 4** shows the spectra of the blue ECDL system with an output power around 350 mW. The laser system is tunable from 454.1 to 456.5 nm for the holographic grating feedback. The bandwidth is around 35 pm in the tunable range, and the ASE is more than 24 dB suppressed. The laser system is tunable from 455.3 to 459.4 nm for the ruled grating feedback; the ASE is more than 18 dB suppressed. The bandwidth (FWHM) is less than 70 pm in the tunable range, but

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With the ruled grating, a further increase of the current increases the spectral bandwidth to a few hundred picometers, and two peaks are present in the spectrum. With the holographic grating, the current can be increased to 0.6 A, where the diode laser is tuned from 455.0 to 456.4 nm with an output power of around 530 mW. The bandwidth is around 63 pm in the tunable range, and the ASE is more than 20 dB suppressed. Further increase of the current also increases the

The output power of the blue ECDL system at different wavelengths is shown in **Figure 5** with these two different gratings. When the holographic grating is applied, the maximum output powers are 85, 198, 368 and 531 mW with injected current of 0.2, 0.3, 0.45 and 0.6 A, respectively. When the ruled grating is applied, the maximum output powers are 90, 222, and 340 mW with injected current of 0.3, 0.6 and 0.9 A, respectively. For both gratings, the output power is relatively constant in the tunable range at each injected current. With the highest output power of the ECDL, that is, 531 mW for the holographic grating feedback and 340 mW for the ruled grating feedback, around 68 and 27% of the output power of the freely running laser are extracted in the ECDL system for the two gratings, respectively. Thus, the efficiency of the ECDL with the holographic grating is much higher than that of the ECDL with the ruled grating. The reason is the higher first-order diffraction efficiency of the ruled grating (53.5%) limits the output power. The tunable range of the blue ECDL system with the ruled grating is much broader than that of the blue ECDL system with the holographic grating when the output power of the diode

**Figure 4.** Optical spectra of the output beam from the ECDL system with (a) a holographic grating at an injected current

of 0.45 A and (b) a ruled grating at an injected current of 0.9 A.

bandwidth to a few hundred picometers for the ECDL with the holographic grating.

side peaks appear in some spectra as shown in **Figure 4(b)**.

**Figure 2.** Optical spectra of the output beam from the blue ECDL system with (a) a holographic grating and (b) a ruled grating. The output power is around 85 mW.

**Figure 3** shows the spectra of the blue ECDL system with an output power around 200 mW with both gratings. **Figure 3(a)** shows seven normalized spectra from 453.7 to 456.5 nm with the holographic grating, at an injected current of 0.3 A. The spectral bandwidth is around 35 pm in the tunable range, that is, two FP modes oscillate simultaneously. **Figure 3(b)** shows the normalized spectra of the ECDL from 453.5 to 459.0 nm with the ruled grating, the injected current is 0.6 A. The bandwidth is less than 67 pm in the tunable range. The ASE suppression is more than 24 dB for the holographic grating feedback and more than 17 dB for the ruled grating feedback.

**Figure 3.** Optical spectra of the output beam from the blue ECDL system with (a) a holographic grating and (b) a ruled grating. The output power is around 200 mW.

**Figure 4** shows the spectra of the blue ECDL system with an output power around 350 mW. The laser system is tunable from 454.1 to 456.5 nm for the holographic grating feedback. The bandwidth is around 35 pm in the tunable range, and the ASE is more than 24 dB suppressed. The laser system is tunable from 455.3 to 459.4 nm for the ruled grating feedback; the ASE is more than 18 dB suppressed. The bandwidth (FWHM) is less than 70 pm in the tunable range, but side peaks appear in some spectra as shown in **Figure 4(b)**.

With the ruled grating, a further increase of the current increases the spectral bandwidth to a few hundred picometers, and two peaks are present in the spectrum. With the holographic grating, the current can be increased to 0.6 A, where the diode laser is tuned from 455.0 to 456.4 nm with an output power of around 530 mW. The bandwidth is around 63 pm in the tunable range, and the ASE is more than 20 dB suppressed. Further increase of the current also increases the bandwidth to a few hundred picometers for the ECDL with the holographic grating.

The output power of the blue ECDL system at different wavelengths is shown in **Figure 5** with these two different gratings. When the holographic grating is applied, the maximum output powers are 85, 198, 368 and 531 mW with injected current of 0.2, 0.3, 0.45 and 0.6 A, respectively. When the ruled grating is applied, the maximum output powers are 90, 222, and 340 mW with injected current of 0.3, 0.6 and 0.9 A, respectively. For both gratings, the output power is relatively constant in the tunable range at each injected current. With the highest output power of the ECDL, that is, 531 mW for the holographic grating feedback and 340 mW for the ruled grating feedback, around 68 and 27% of the output power of the freely running laser are extracted in the ECDL system for the two gratings, respectively. Thus, the efficiency of the ECDL with the holographic grating is much higher than that of the ECDL with the ruled grating. The reason is the higher first-order diffraction efficiency of the ruled grating (53.5%) limits the output power.

**Figure 3** shows the spectra of the blue ECDL system with an output power around 200 mW with both gratings. **Figure 3(a)** shows seven normalized spectra from 453.7 to 456.5 nm with the holographic grating, at an injected current of 0.3 A. The spectral bandwidth is around 35 pm in the tunable range, that is, two FP modes oscillate simultaneously. **Figure 3(b)** shows the normalized spectra of the ECDL from 453.5 to 459.0 nm with the ruled grating, the injected current is 0.6 A. The bandwidth is less than 67 pm in the tunable range. The ASE suppression is more than 24 dB for the holographic grating feedback and more than 17 dB for the ruled

**Figure 3.** Optical spectra of the output beam from the blue ECDL system with (a) a holographic grating and (b) a ruled

**Figure 2.** Optical spectra of the output beam from the blue ECDL system with (a) a holographic grating and (b) a ruled

grating feedback.

grating. The output power is around 85 mW.

6 Laser Technology and its Applications

grating. The output power is around 200 mW.

The tunable range of the blue ECDL system with the ruled grating is much broader than that of the blue ECDL system with the holographic grating when the output power of the diode

**Figure 4.** Optical spectra of the output beam from the ECDL system with (a) a holographic grating at an injected current of 0.45 A and (b) a ruled grating at an injected current of 0.9 A.

The tunability of the green ECDL system operated in both *s*- and *p*-polarized modes is characterized by measuring the optical spectrum of the output beam at different wavelengths. The spectra of the ECDL for both operating modes with an operating current of 330 mA are shown in **Figure 6**. The output power of the green ECDL system for the *s*-polarized and *p*-polarized mode operation are around 70 and 50 mW, respectively. **Figure 6(a)** shows eight normalized spectra from 511.1 to 516.0 nm for *s*-polarized mode operation. The spectral bandwidths of these spectra are around 8 pm. Compared with the 39 pm mode spacing of the FP modes, the ECDL in the *s*-polarized mode operates in single FP mode. **Figure 6(b)** shows 10 spectra from 508.8 to 518 nm for *p*-polarized mode operation. The bandwidth is less than 9 pm in the tunable range. The ASE is more than 15 and 17 dB suppressed for *s*- and *p*-polarized mode

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**Figure 7** shows the optical spectra of the green ECDL system for both operation modes with the output power around 290 mW. To achieve the 290 mW output power, 600 and 850 mA injected currents are needed for *s*- and *p*-polarized mode operation, respectively. Six normalized spectra from 513.1 to 515.7 nm are shown in **Figure 7(a)** for *s*-polarized mode operation. Two peaks are observed in the spectra and the spectral bandwidth is less than 0.25 nm. Five spectra from 512.9 to 517.5 nm are shown in **Figure 7(b)** for *p*-polarized mode operation. The spectral bandwidth (FWHM) is less than 50 pm in the tunable range, however side modes appear in the spectra. The ASE is more than 15 dB suppressed in the tunable ranges for both *s*and *p*-polarized mode operation. In the *s*-polarized mode, the injected current can be further increased to 850 mA, and the output power is increased to 480 mW. The ECDL system can be tuned from 514.1 to 516.2 nm, the emission bandwidth is less than 0.5 nm in the tunable range,

**Figure 8** shows the output power of the ECDL system at different wavelengths and injected currents for both operating modes. In the *s*-polarized mode, the maximum output powers

**Figure 6.** Optical spectra of green ECDL system operates in (a) *s*-polarized mode and (b) *p*-polarized mode. The injected

operation, respectively.

and the ASE is more than 15 dB suppressed.

current is 330 mA for both conditions.

**Figure 5.** Output power of the blue ECDL at different wavelengths and operating currents with the holographic grating (red signs) and the ruled grating (black signs).

laser system is comparable. The reason is that the first-order diffraction efficiency of the ruled grating is much higher compared with that of the holographic grating, that is, 53.5 versus 8.3%. The higher the feedback strength from the grating, the more effective the suppression of the freely running lasing modes.

In summary, a high-power, tunable, narrow-bandwidth ECDL system based on a GaN diode laser around 455 nm is demonstrated. The laser system can be tuned over 1.4 nm with an output power around 530 mW when the holographic grating is applied; the laser system can be tuned over 6.0 nm with an output power of 80 mW when the ruled grating is applied. The results show the efficiency of the ECDL with holographic grating is higher, but the tunable range of the ECDL with ruled grating is broader.

### **3.2. Green GaN external-cavity diode laser system**

In this subsection, the results of a green ECDL system are presented [16]. For the green ECDL system, only the holographic grating (Thorlabs, GH13-24 U) is applied. A half-wave plate is inserted in the external cavity to turn the polarization direction of the laser beam. Thus, the green laser system can be operated in both *s*-polarized mode (the laser beam is polarized along the lines of the grating) and *p*-polarized mode (the laser beam is polarized perpendicular to the lines of the grating). The zeroth- and first-order diffraction efficiencies of the grating are around 81 and 7% for the *s*-polarized beam, and around 48 and 29% for the *p*-polarized beam, respectively. The length of the external cavity of the laser system is around 140 mm, corresponding to a 0.95 pm external-cavity mode spacing.

The threshold current of the freely running green diode laser is around 0.25 A, and the slope efficiency is around 1.0 W/A. With an injected current of 1.4 A, 1.1 W output power is obtained. The laser diode is operating in multiple FP modes centered around 515 nm, with a spectral bandwidth of 1.3 nm. The mode spacing of the FP modes is around 39 pm.

The tunability of the green ECDL system operated in both *s*- and *p*-polarized modes is characterized by measuring the optical spectrum of the output beam at different wavelengths. The spectra of the ECDL for both operating modes with an operating current of 330 mA are shown in **Figure 6**. The output power of the green ECDL system for the *s*-polarized and *p*-polarized mode operation are around 70 and 50 mW, respectively. **Figure 6(a)** shows eight normalized spectra from 511.1 to 516.0 nm for *s*-polarized mode operation. The spectral bandwidths of these spectra are around 8 pm. Compared with the 39 pm mode spacing of the FP modes, the ECDL in the *s*-polarized mode operates in single FP mode. **Figure 6(b)** shows 10 spectra from 508.8 to 518 nm for *p*-polarized mode operation. The bandwidth is less than 9 pm in the tunable range. The ASE is more than 15 and 17 dB suppressed for *s*- and *p*-polarized mode operation, respectively.

**Figure 7** shows the optical spectra of the green ECDL system for both operation modes with the output power around 290 mW. To achieve the 290 mW output power, 600 and 850 mA injected currents are needed for *s*- and *p*-polarized mode operation, respectively. Six normalized spectra from 513.1 to 515.7 nm are shown in **Figure 7(a)** for *s*-polarized mode operation. Two peaks are observed in the spectra and the spectral bandwidth is less than 0.25 nm. Five spectra from 512.9 to 517.5 nm are shown in **Figure 7(b)** for *p*-polarized mode operation. The spectral bandwidth (FWHM) is less than 50 pm in the tunable range, however side modes appear in the spectra. The ASE is more than 15 dB suppressed in the tunable ranges for both *s*and *p*-polarized mode operation. In the *s*-polarized mode, the injected current can be further increased to 850 mA, and the output power is increased to 480 mW. The ECDL system can be tuned from 514.1 to 516.2 nm, the emission bandwidth is less than 0.5 nm in the tunable range, and the ASE is more than 15 dB suppressed.

laser system is comparable. The reason is that the first-order diffraction efficiency of the ruled grating is much higher compared with that of the holographic grating, that is, 53.5 versus 8.3%. The higher the feedback strength from the grating, the more effective the suppression of

**Figure 5.** Output power of the blue ECDL at different wavelengths and operating currents with the holographic grating

In summary, a high-power, tunable, narrow-bandwidth ECDL system based on a GaN diode laser around 455 nm is demonstrated. The laser system can be tuned over 1.4 nm with an output power around 530 mW when the holographic grating is applied; the laser system can be tuned over 6.0 nm with an output power of 80 mW when the ruled grating is applied. The results show the efficiency of the ECDL with holographic grating is higher, but the tunable

In this subsection, the results of a green ECDL system are presented [16]. For the green ECDL system, only the holographic grating (Thorlabs, GH13-24 U) is applied. A half-wave plate is inserted in the external cavity to turn the polarization direction of the laser beam. Thus, the green laser system can be operated in both *s*-polarized mode (the laser beam is polarized along the lines of the grating) and *p*-polarized mode (the laser beam is polarized perpendicular to the lines of the grating). The zeroth- and first-order diffraction efficiencies of the grating are around 81 and 7% for the *s*-polarized beam, and around 48 and 29% for the *p*-polarized beam, respectively. The length of the external cavity of the laser system is around 140 mm,

The threshold current of the freely running green diode laser is around 0.25 A, and the slope efficiency is around 1.0 W/A. With an injected current of 1.4 A, 1.1 W output power is obtained. The laser diode is operating in multiple FP modes centered around 515 nm, with a spectral

the freely running lasing modes.

(red signs) and the ruled grating (black signs).

8 Laser Technology and its Applications

range of the ECDL with ruled grating is broader.

**3.2. Green GaN external-cavity diode laser system**

corresponding to a 0.95 pm external-cavity mode spacing.

bandwidth of 1.3 nm. The mode spacing of the FP modes is around 39 pm.

**Figure 8** shows the output power of the ECDL system at different wavelengths and injected currents for both operating modes. In the *s*-polarized mode, the maximum output powers

**Figure 6.** Optical spectra of green ECDL system operates in (a) *s*-polarized mode and (b) *p*-polarized mode. The injected current is 330 mA for both conditions.

mode operation. However, **Figure 8** shows the tunable range of the ECDL in *p*-polarized mode operation is much broader than that of the ECDL in the *s*-polarized mode operation. The main reason for this difference is the higher zeroth-order diffractive efficiency of the holographic grating for the *s*-polarized beam compared with that for the *p*-polarized beam, that is, 81 versus 48%. The higher zeroth-order diffraction efficiency of the grating for *s*-polarized beam means a high output coupling efficiency, that is, high output power. However, the first-order diffractive efficiency for the *p*-polarized beam is much higher than that for the *s*-polarized beam, that is, 29 versus 7%. The higher the feedback strength from the grating (the higher first-order diffraction efficiency), the more effective the suppression of the freely running lasing mode, thus the tun-

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In summary, a high-power, tunable, narrow-bandwidth ECDL system based on a GaN device around 515 nm is demonstrated. The laser system can be tuned over 2.1 nm with an output power around 480 mW in the *s*-polarized mode operation; the laser system can be tuned over 9.0 nm with an output power of 50 mW in the *p*-polarized mode operation. We can choose different operating modes for different priorities, that is, high-power or broad tuning range,

Both the blue and green tunable ECDL systems show the efficiency (output power) of the ECDL system is higher when the zeroth-order diffraction efficiency of the grating used in the laser system is higher, such as the conditions for the holographic grating in the blue diode laser system and the *s*-polarized mode operation for the green diode laser system. However, the tunable range of the ECDL system with higher efficiency is narrower since a higher firstorder diffraction efficiency of the grating is needed to achieve a wider tunable range. Thus, there is a compromise between the output power and the tunable range of the ECDL system

To understand this compromise between the efficiency (output power) and tunable range further, a theoretical analysis is given below based on rate equations of the diode laser. The rate equations describing the carrier density *N* and the photon density *N*ph in the diode laser

> *N*ph *τ*ph

of an electron, *d* is the thickness of the active region, *τ* is the carrier lifetime, *v*gr is the group velocity of the photons, *g*(*N*) is the material gain, *Г* is the confinement factor, *τ*ph is the photon lifetime, *α*m is the mirror loss, *K* is the feedback strength of the grating (the first-order diffrac-

+ *α*<sup>m</sup> *v*gr *KN*ph(*t* − *t*

*<sup>τ</sup>* − *v*gr *g*(*N*) *N*ph, (1)

is the internal efficiency, *q* is the elementary charge

0

0), (2)

is the time delay of

since the sum of the zeroth- and first-order diffraction efficiencies is around unity.

*dt* <sup>=</sup> *<sup>η</sup>*<sup>i</sup> *<sup>j</sup>* \_\_\_

*dt* <sup>=</sup> *<sup>v</sup>*gr <sup>Γ</sup>*g*(*N*) *<sup>N</sup>*ph <sup>−</sup> \_\_\_

tion efficiency in our case, here the other loss in the cavity is neglected.), *t*

*qd* <sup>−</sup> \_\_ *N*

able range of the green ECDL system operated in the *p*-polarized mode is broader.

using only one ECDL system.

cavity are given as [15, 17]:

\_\_\_\_

\_\_\_ *dN*

where *j* is the inject current density, *η*<sup>i</sup>

*dN*ph

**3.3. Selection of diffraction grating for ECDL**

**Figure 7.** Optical spectra of the green ECDL system operates in (a) *s*-polarized mode and (b) *p*-polarized mode. The output power is around 290 mW for both operation modes.

**Figure 8.** Output power of the ECDL at different wavelengths and operating currents, operated in *s*-polarized mode (red signs) and *p*-polarized mode (black signs).

of the ECDL are 75, 173, 296 and 481 mW with injected current of 330, 450, 600 and 850 mA, respectively. In the *p*-polarized mode, the maximum output powers are 56, 161 and 293 mW with injected current of 330, 550 and 850 mA, respectively. The output power is relatively constant in the tunable range at each injected current for both operating modes.

With the highest injected current, that is, 850 mA, the maximum output powers for the *s*- and *p*-polarized mode operation are 481 and 293 mW. This means that 75 and 46% of the output power in freely running condition is extracted in the ECDL system for the *s*- and *p*-polarized mode operation. However, **Figure 8** shows the tunable range of the ECDL in *p*-polarized mode operation is much broader than that of the ECDL in the *s*-polarized mode operation. The main reason for this difference is the higher zeroth-order diffractive efficiency of the holographic grating for the *s*-polarized beam compared with that for the *p*-polarized beam, that is, 81 versus 48%. The higher zeroth-order diffraction efficiency of the grating for *s*-polarized beam means a high output coupling efficiency, that is, high output power. However, the first-order diffractive efficiency for the *p*-polarized beam is much higher than that for the *s*-polarized beam, that is, 29 versus 7%. The higher the feedback strength from the grating (the higher first-order diffraction efficiency), the more effective the suppression of the freely running lasing mode, thus the tunable range of the green ECDL system operated in the *p*-polarized mode is broader.

In summary, a high-power, tunable, narrow-bandwidth ECDL system based on a GaN device around 515 nm is demonstrated. The laser system can be tuned over 2.1 nm with an output power around 480 mW in the *s*-polarized mode operation; the laser system can be tuned over 9.0 nm with an output power of 50 mW in the *p*-polarized mode operation. We can choose different operating modes for different priorities, that is, high-power or broad tuning range, using only one ECDL system.
