Single-Frequency External Cavity Green Diode Laser

We present a design of a single-frequency green InGaN external cavity diode laser (ECDL) with 40-mW output power and 10-MHz linewidth. The ECDL was mounted in the Littrow configuration, with an overall efficiency of approximately 52% and a tuning region of 8 nm. We analyzed the output efficiency of the ECDL under different polarization directions and emitted light patterns. We used a spectrometer to analyze the laser spectrum under the different applied currents and temperatures. Finally, we obtained the linewidth of the ECDL using a scanning Fabry-Perot interferometer and found that the linewidth varied with the wavelength.

SECTION 1. Introduction

Laser diodes (LDs) are widely used in applications such as laser ranging, laser cutting, fiber sensing, fiber communication, etc. However, free-running LDs suffer from the problem of mode-hopping, with laser frequencies that are very sensitive to the operation environment. Their frequency may be affected by the environmental temperature and injection current, because of the short cavity and temperature-sensitive refractive index. Although the lasing mechanism of diode lasers is regarded as homogenous broadening, the bandwidth of the gain profile is very large, in the range of hundreds of megahertz to tens of gigahertz. To obtain single-frequency laser output, several arrangements have added dispersive elements, such as a diffraction grating outside the diode laser that offers an external cavity and frequency-selective optical feedback. This type of the cavity design is called as an external cavity diode laser (ECDL) [1].

ECDLs are mainly composed of a semiconductor laser and a diffraction grating. The frequency-selective optical feedback, enabled by adjusting the angle of the diffraction grating, can provide single-longitudinal-mode operation. Two methods, the Littrow [2], [3] and Littman–Metcalf configurations [4], [5] , are commonly used. In these two configurations, the first-order diffraction beam from the grating is used as the optical feedback signal, and the zeroth-order diffraction beam is the laser output. The difference between the two configurations is the optical feedback mechanism. The first-order diffraction beam of the Littrow configuration is directly coupled back into the LD. However, in the Littman–Metcalf configuration, the first-order diffraction beam is generated by the grating and then reflected back to the LD by an addition mirror. By tuning the angle of the diffraction grating and mirror in the Littrow and Littman–Metcalf configurations, respectively, the wavelength selection can be achieved. Owing to its simpler structure, the Littrow configuration is more commonly used for ECDL systems.

Traditionally, a green laser can be obtained by using an intra-cavity frequency-doubled design. Diode-pumped solid-state lasers can emit 532-nm green beams in the submilliwatt to tens of watts range [6], [7]. In recent years, Fabry–Perot-type green LDs have been reported. Avramescu and coworkers demonstrated a direct green diode laser that emits at 524 nm with 50 mW output power [8]. There are numerous applications based on green lasers, including pump source, sensing, ranging, and undersea communications. In the future, it is possible that frequency-converted green lasers will be replaced by direct emitting lasers based on InGaN compound semiconductors. The narrow-linewidth single-frequency lasers have been addressed for the studies on the phase change metamaterials [9]–​[11] , and on an all-optical tunable Fano resonance [12] due to high precise phase control.

In this work, we implement a single-frequency green ECDL system based on Littrow configuration. At first, we introduce the ECDL design and the experimental scheme. We measured the optical spectra, power–current (L–I) curves, and the linewidth of the ECDL system, and the results are presented.

SECTION 2. Experimental Details

The experimental setup is schematically illustrated in Fig. 1. The ECDL system consists of a green LD, a collimation lens, a half-wave plate (HWP) and a grating. The LD (Thorlabs L520P120) was mounted in an adjustable collimation tube (Thorlabs LTN-330A). A low-noise quantum cascade laser driver (Wavelength electronics QCL500 LAB) supplied the current to the diode laser, and the light passed through the HWP and was incident on the grating (Thorlabs GR13-1850) with 1800 grooves/mm. The first-order diffraction beam was coupled back to the diode laser as the feedback signal, and the zeroth-order diffraction beam was used as the output of the ECDL. The grating was attached to a piezo actuator (PZT, Thorlabs TLK-PZT1) on the mirror mount for adjusting the output wavelength. The external cavity length, from the LD to the grating, was approximately 30 mm. The PZT under the grating was used to adjust the cavity length for fine frequency tuning. The ECDL system was mounted in an aluminum box. This aluminum box was temperature controlled by a temperature controller (Thorlabs TED200C) and sensor (AD590). We kept the room temperature as 25 °C, and mounted the ECDL system on an air-floated optical table to reduce the mechanical vibration.

Fig. 1.

Experimental scheme of the external cavity green diode laser. HWP: half-wave plate; QWP: quarter-wave plate; SFPI: scanning Fabry–Perot interferometer; PZT: piezo actuator.

We measured the output spectra of the LD and the ECDL system with a spectrometer. Then, we used a scanning Fabry–Perot interferometer (SFPI) to check the linewidth of the ECLD single frequency output. The mechanism of the ECDL system is described in Supplemental Information A. The HWP between the LD and the grating was used to adjust the LD polarization direction.

SECTION 3. Results

3.1 Single-Longitudinal-Mode Output From External Cavity Diode Laser

The spectra of the LD under the applied currents of 120, 160, and 260 mA were measured and shown in Fig. 2(a), with the LD temperature set to 20 °C. There is only a single longitudinal mode at low applied current, just above the lasing threshold. The number of modes increases with the applied current. The spectra of the LD and ECDL at the same applied current were recorded, as shown in Fig. 2(b). The single longitudinal mode was picked up in the ECDL. We then attempted to determine the effects that can impact the output beam quality.

Fig. 2.

(a) Lasing spectra of the diode laser with the applied currents of 120, 160, and 260 mA (T = 20°C). (b) Lasing spectra of direct LD and ECDL with the applied current of 260 mA (T = 20 °C).

3.2 Output Power of ECDL With Different Light Patterns

The diode laser was mounted in the collimation tube, and then the output beam was adjusted to focus on a wall, at a distance of approximately 4 m. The output beam was elliptical. We directly rotated the collimation tube, so that the long axis of the elliptical laser beam could be perpendicular (horizontal pattern) or parallel (vertical pattern) to the grating grooves, as shown in Fig. 3.

Fig. 3.

Horizontal and vertical light patterns on the grating.

We measured the output power of the ECDL for the two light patterns, and the results are shown in Fig. 4(a). The output efficiencies of the horizontal and vertical light patterns on the grating are approximately 52% and 3%, respectively. Furthermore, we found that the threshold current decreases in the ECDL system. This means that we can use a lower injection current to achieve lasing. Fig. 4(b) shows the L–I curve measured at 25 °C. The power stability determined the laser quality. When the ECDL was operated at 300-mA injection current and 25 °C for 4-hour free running time, the instability of the output power is less than 3.4%.

Fig. 4.

(a) L–I curve of different light patterns on the grating at 25 °C. (b) L–I curve of diode laser and ECDL with low injection current at 25 °C.

The HWP between the diode laser and the diffraction grating was used to adjust the polarization orientation of the diode laser. Initially, we rotated the collimation tube such that the long axis of the elliptical laser beam was perpendicular to the grating lines (horizontal light pattern). Then, we rotated the HWP and measured the output power of the ECDL, yielding the results shown in Fig. 5. Generally, LDs are linearly polarized and the polarization direction is perpendicular to the long axis of the elliptical laser beam, owing to its construction. The results clearly show that the polarization direction that is parallel to the grating grooves is more efficient than the polarization direction that is perpendicular to the grating. The perpendicular polarization has a higher diffraction efficiency than the parallel polarization at 520 nm. Because most of the perpendicularly polarized light was reflected back to the diode laser, the output power was relatively small. Therefore, we can tune the output power by controlling the polarization direction of the diode laser.

Fig. 5.

Variance of output power with HWP rotation at 25 °C and 250 mA.

3.3 Wavelength Tuning Range of ECDL With Applied Currents and Temperatures

We used a spectrometer to obtain the whole spectra from the ECDL, and determined the output wavelength. The spectra can also give us the linewidth information, but the resolution is too low to obtain the actual linewidth. The results are shown in Fig. 6. The characteristic peaks were obtained by fitting a Lorentzian profile. We found that the peak wavelengths were insensitive to the applied currents, at approximately 518.3 nm. The variation due to current tuning was calculated as 0.033 nm.

Fig. 6.

(a) Spectra of ECDL at 20 °C from 120 to 260 mA. (b) Peak wavelength with applied currents.

The peak wavelength from the ECDL could be tuned by rotating one of the axes of the mirror mount. Therefore, we set the temperature at 25 °C and observed the range at different operation currents, as shown in Fig. 7. When the ECDL operated at a threshold current of ${\rm{I}}{\text{th}}\,= \,122\,{\rm{mA}}$, the wavelength range was approximately 10.5 nm, from 512.53 nm to 523.03 nm. As the current increased to 2.5Ith (305 mA), the range reduced to approximately 4.9 nm, i.e., 514.68–519.59 nm. These results were similar to those obtained previously [13]. We think that the wider spectra occur near the lasing threshold due to weak mode competition. Compared with direct LDs, the ECDL design lowers the lasing threshold since the grating provides larger feedback than the laser output facet.

Fig. 7.

Wavelength-tunable range under different applied currents.

3.4 Linewidth Analysis of ECDL With Different Applied Currents and Temperatures

For specific applications, such as precision measurements of atomic transitions, or hyperfine iodine transitions, the linewidth of the ECDL output is regarded as the quality parameter of the ECDL. The free spectral range of our SFPI (Thorlabs SA200-3B) is 1.5 GHz, and its finesse is 246. The detected linewidth limit is estimated as 6 MHz. Fig. 8(a) shows the signal obtained with the oscilloscope. The frequency difference between two adjacent peaks is 1.5 GHz. We obtained the linewidth by fitting these data to a Gaussian profile. Fig. 8(b) shows the linewidth of the ECDL for different operating currents, at temperatures of 15, 20, and 25 °C. The linewidth is approximately 10.3 MHz. We also found that the linewidth decreased to 6.9 MHz when the wavelength was pushed to the edge of the tuning range. The linewidth broadening could be induced by the vibration, and the instability of current source and LD temperature.

Fig. 8.

(a) Signal of the ECDL by SFPI. (b) Linewidth of the ECDL for different currents and temperatures.

SECTION 4. Conclusion

Single-frequency lasers have good coherence, and they can be used as light sources for high-precision interferometers or optical spectroscopic applications. In our experiment, we implemented a temperature-controlled ECDL emitting green light. The system has an output power of approximately 42 mW and a linewidth of 10 MHz at twice the threshold current at 25 °C. Furthermore, the ECDL efficiency is influenced by the LD light pattern and light polarization. Here, we found that the diode laser has a single longitudinal mode when operating in low applied current. As the injection current increases, more modes appear, which potentially compete with each other. In our simple ECDL system, the single longitudinal mode was selected and reflected to the diode laser as feedback. The spectra of the ECDL show that the ECDL output wavelength can be maintained at 518.3 nm at the highest output power. The tunable range can be extended to 10.5 nm when operating at the threshold current. As the applied current is increased to twice the threshold current, the tunable range decreases to 5 nm. With a proper arrangement, the ECDL demonstrated in our experiment has a slope efficiency of 0.649 mW/mA at 25 °C. The maximum efficiency of this ECDL is 52%. In the future, we will study the iodine hyperfine spectrum using our ECDL.