Mingjun Chi

Tunable high-power narrow-linewidth semiconductor laser based on an external-cavity tapered amplifier

A high-power narrow-linewidth laser system based on a tapered semiconductor optical amplifier in external cavity is demonstrated. The external cavity laser system uses a new tapered amplifier with a super-large optical-cavity (SLOC) design that leads to improved performance of the external cavity diode lasers. The laser system is tunable over a 29 nm range centered at 802 nm. As high as 1.95 W output power is obtained at 803.84 nm, and an output power above 1.5 W is achieved from 793 to 812 nm at operating current of 3.0 A. The emission linewidth is below 0.004 nm and the beam quality factor M2 is below 1.3 over the 29 nm tunable range. As an example of application, the laser system is used as a pump source for the generation of 405 nm blue light by single-pass frequency doubling in a periodically poled KTiOPO4. An output power of 24 mW at 405 nm, corresponding to a conversion efficiency of 0.83%/W is attained.

High brightness laser source based on polarization coupling of two diode lasers with asymmetric feedback

In this letter, we show that polarization coupling and asymmetric diode-laser feedback can be used to combine two diode-laser beams with low spatial coherence into a single beam with high spatial coherence. The coupled laser source is based on two similar laser systems each consisting of a 1 μm×200 μm broad area laser diode applied with a specially designed feedback circuit. When operating at two times threshold, 50% of the freely running system output power is obtained in a single beam with an M2 beam quality factor of 1.6±0.1, whereas the M2 values of the two freely running diode lasers are 29±1 and 34±1, respectively.

Self-injection locking of an extraordinarily wide broad-area diode laser with a 1000-μm-wide emitter

The experimental results of self-injection locking of an antireflection-coated broad-area diode laser with a 1000-microm-wide emitting area are presented. To our knowledge, it is the broadest single-element diode laser that has been used in an external-feedback cavity until now. Usually, wide diode lasers suffer from filamentation, which leads to poor spatial beam quality. We show, however, that the beam quality of the diode laser is improved significantly when we use asymmetric self-injection locking. An output power of 2.05 W is obtained with a beam quality factor M2 of 2.7. The self-injection locking technique improves the beam quality by a factor of 107. By comparing the results with those obtained with an ordinarily coated diode laser with a 1000-microm-wide emitter we show that antireflection coating on the front facet is decisive for this improvement in the beam quality.

High-power dual-wavelength external-cavity diode laser based on tapered amplifier with tunable terahertz frequency difference.

Tunable dual-wavelength operation of a diode laser system based on a tapered diode amplifier with double-Littrow external-cavity feedback is demonstrated around 800 nm. The two wavelengths can be tuned individually, and the frequency difference of the two wavelengths is tunable from 0.5 to 5.0 THz. An output power of 1.54 W is achieved with a frequency difference of 0.86 THz, the output power is higher than 1.3 W in the 5.0 THz range of frequency difference, and the amplified spontaneous emission intensity is more than 20 dB suppressed in the range of frequency difference. To our knowledge, this is the highest output power from a dual-wavelength diode laser system operating with tunable terahertz frequency difference.

A general theory of two-wave mixing in nonlinear media

A general theory of two-wave mixing (TWM) in nonlinear media is presented. Assuming a gain (or absorption) grating and a refractive index grating are generated because of the nonlinear process in a nonlinear medium, the coupled-wave equations of TWM are derived based on Maxwells wave equation. The coupled-wave equations can be decoupled as coupled equations for the intensity and coupled equations for the phase of both beams, and these two sets of coupled equations can be solved analytically by using average total intensity in the medium instead of using the total intensity. Compared with the previous theory of TWM, the theory presented here is more general, and the applications of the theory to photorefractive materials, Kerr media, and semiconductor broad-area amplifiers are described.

Improvement of the beam quality of a broad-area diode laser using double feedback from two external mirrors

In this letter, a symmetric double-feedback configuration, to improve the beam quality of broad-area diode lasers is demonstrated. With this configuration, a symmetric double-lobed far field can be obtained, and this configuration leads to good beam quality. The beam quality factor M2 of a diode laser with the emitting area 1μm×200μm is improved by using both the asymmetric single feedback and the symmetric double feedback. M2 values of 4.3 for the asymmetric single-feedback laser system and 3.3 for the symmetric double-feedback laser system are obtained, whereas the M2 value of the freely running laser is 42. The far and the near fields are also measured and compared for the three conditions.

How does external feedback cause AlGaAs-based diode lasers to degrade?

The effect of external feedback on the degradation of 808 nm emitting AlGaAs-based high-power broad-area diode lasers is studied. For this purpose, early stages of gradual degradation are induced by accelerated aging at high power levels. While the quantum well that actually experiences the highest total optical load remains unaffected, severe impact by point defects is observed on the cladding layers and the waveguide. Extended defects such as dislocations, however, are not observed in such early stages of degradation, which are accompanied by gradual power loss of a few percent only.

Tuning range and output power optimization of an external-cavity GaN diode laser at 455 nm

In this paper we discuss how different feedback gratings affect the tuning range and the output power of external feedback diode laser systems. A tunable high-power narrow-spectrum external-cavity diode laser system around 455 nm is investigated. The laser system is based on a high-power GaN diode laser in a Littrow external-cavity. Both a holographic diffraction grating and a ruled diffraction grating are used as feedback elements in the external cavity. The output power, spectral bandwidth, and tunable range of the external cavity diode laser system are measured and compared with the two gratings at different injected currents. When the holographic grating is used, the laser system can be tuned over a range of 1.4 nm with an output power around 530 mW. When the ruled grating is used, the laser system can be tuned over a range of 6.0 nm with an output power around 80 mW. The results can be used as a guide for selecting gratings for external-cavity diode lasers for different requirements.

Nonlinear gain amplification due to two-wave mixing in a broad-area semiconductor amplifier with moving gratings

Two-wave mixing in a broad-area semiconductor amplifier with moving gratings is investigated. It is shown that depending on direction of the moving gratings and the anti-guiding parameter the optical gain may increase or decrease.

Tunable high-power narrow-spectrum external-cavity diode laser based on tapered amplifier at 668 nm.

A 668 nm tunable high-power narrow-spectrum diode laser system based on a tapered semiconductor optical amplifier in external cavity is demonstrated. The laser system is tunable from 659to675 nm. As high as 1.38 W output power is obtained at 668.35 nm. The emission spectral bandwidth is less than 0.07 nm throughout the tuning range, and the beam quality factor M(2) is 2.0 with the output power of 1.27 W.