FBH research: 09.11.2017

High-power Y-branch MOPA system with 9.7 nm combined wavelength tunability

Sketch of the hybrid dual-wavelength MOPA system
Fig. 1: Sketch of the hybrid dual-wavelength MOPA system
False color contour plot of the wavelength tuning
Fig. 2: False color contour plot of the wavelength tuning of the individual arms (super imposed) as function of the heater current. Inset: example of the side-mode-suppresion-ratio
Optical power versus the emission wavelength
Fig. 3: Optical power versus the emission wavelength for the two arms

Tunable diode laser sources in the near infrared (NIR) are needed in several applications, such as absorption spectroscopy, bio-medical imaging, terahertz frequency generation, and particularly in frequency conversion applications. Common for these applications is the need for high power, diffraction limited, and tunable narrow linewidth laser sources.

A light source fulfilling these characteristics was recently developed at FBH, utilizing a tunable Y-branch (dual wavelength) hybrid master oscillator power amplifier (MOPA). The MO is a monolithic Y-branch distributed Bragg reflector (DBR) diode laser, which is collimated and coupled into a tapered power amplifier (TPA) using cylindrical micro-lenses in a compact 25 mm × 25 mm conduction cooled laser package, see Fig. 1.spectively.

The MO emits at the two wavelengths: λLeft = 973.67 nm and lRight = 975.91 nm, with a spectral resolution smaller than 17 pm. The PA ensures similar amplification from both branches, with output powers of PLeft = 5.4 W and PRight = 5.5 W, respectively.

The emitted light has a beam waist of 6.4 µm (1/e2) and a far field angle of 13.7° (1/e2), resulting in a propagation factor of M21/e2 = 2.2 along the slow axis, and a power content of 72% within the central lobe. This corresponds to a diffraction limited output power of approximately 4 W.

The wavelength tuning of this device is obtained by using an electrically controlled micro-heater, implemented on top of the grating sections of the MO. The left arm can be tuned from ΔλLeft = [973.70–981.20] nm and the right arm from ΔλRight = [975.89–983.40] nm, see Fig. 2. This corresponds to a wavelength tuning of Δλ = 7.5 nm from each arm. With a spectral distance of 2.23 nm between the two arms, and in combination with the micro-heater, a total combined wavelength tuning of 9.7 nm is thus obtained. These results indicate that with proper wavelength distancing of each arm, and possibly the use of NArms of tunable seed lasers, a total combined tunability of about 7.5 nm x NArms can be achieved.

The output power of the MOPA system as function of the wavelength is shown in Fig. 3. The left arm emits an average power of PLeft = 5.41 W ± 0.03 W, corresponding to a power variation of ± 0.5%. Similarly, the right arm emits an average output power of PRight = 5.48 W ± 0.01 W with a variation of only ± 0.2% during wavelength tuning.

Overall, the combination of high power, tunable narrow spectral width, compactness, and stability makes this device ideal for applications including nonlinear frequency conversion.

This work was partly carried out within the Mid-TECH project funded by the European Union’s Horizon 2020 program under grant agreement No. 642661.

Publication

M. Tawfieq, A. Müller, J. Fricke, P. Della Casa, P. Ressel, D. Feise, B. Sumpf, and G. Tränkle, "Extended 9.7 nm tuning range in a MOPA system with a tunable dual grating Y-branch laser", Opt. Lett. 42(20), 4227-4230 (2017).