T. Jouhti

Mode-locked ytterbium fiber laser tunable in the 980–1070-nm spectral range

Spectral tuning of a mode-locked Yb-doped fiber laser over a 90-nm range is reported. Using semiconductor saturable absorber mirrors in a fiber laser cavity incorporating a grating-pair dispersive delay line, we obtain reliable self-starting mode locking over the whole tuning range. The wide tuning range is achieved by optimization of reflection characteristics and bandgap energy of the multiple-quantum-well semiconductor saturable absorber and by proper engineering of the laser cavity.

Low-threshold-current 1.32-μm GaInNAs/GaAs single-quantum-well lasers grown by molecular-beam epitaxy

Using solid-source molecular-beam epitaxy with a rf-plasma source, we have grown GaInNAs/GaAs single-quantum-well lasers operating at 1.32 μm. For a broad-area oxide stripe, uncoated Fabry–Perot laser with a cavity length of 1600 μm, the threshold current density is 546 A/cm2 at room temperature. The internal quantum efficiency for these lasers is 80%, while the materials losses are 7.0 cm−1. A characteristic temperature of 104 K was measured in the temperature range from 20 to 80 °C. Optical output up to 40 mW per facet under continuous-wave operation was achieved for these uncoated lasers at room temperature.

Picosecond SESAM-based ytterbium mode-locked fiber lasers

Using semiconductor saturable absorber mirrors and a grating-pair dispersion compensator, we obtain reliable self-starting mode locking of a ytterbium (Yb) fiber laser tunable over 125 nm. The 980-1105-nm tuning range is achieved by optimization of nonlinear reflection and bandgap characteristics of the multiple-quantum-well saturable absorber and by proper engineering of the laser cavity. A short-length Yb-doped double-clad amplifier seeded with mode-locked Yb-fiber laser produces picosecond pulses with energy of 30 nJ (700 mW of average power). A compact version of the fiber laser was built using a Gires-Tournois compensator and short length (1-cm long) of highly doped Yb fiber. Using a novel semiconductor saturable abserver mirror based on GaInNAs structure, self-started 1.5-ps pulse mode-locked operation was obtained at 1023 nm with a repetition rate of 95 MHz. A mode-locked Yb-doped fiber laser was also developed without using any dispersion compensation technique. Overall group-velocity dispersion was minimized by using highly doped Yb fiber in a compact amplifying loop cavity. Self-started mode-locked operation was obtained in 980-1030-nm wavelength range with a fundamental repetition rate of 140 MHz. Without using dispersion compensation, the lasers produced pulses in a range from 15 to 26 ps.

1.5-µm monolithic GaInNAs semiconductor saturable-absorber mode locking of an erbium fiber laser

We present a new monolithic GaAs-based semiconductor saturable absorber operating at 1.55 microm. An epitaxially grown absorber mirror in a GaInNAs/GaAs material system was successfully used to mode lock an erbium-doped fiber laser. The GaInNAs material system possesses intriguing physical properties and provides great potential for lasers and nonlinear optical devices operating at the 1.3-1.55-microm wavelength range.

Effects of insertion of strain-mediating layers on luminescence properties of 1.3-μm GaInNAs/GaNAs/GaAs quantum-well structures

We present a 1.3-μm GaInNAs/GaAs quantum-well heterostructure, which consists of a strain-mediating GaInNAs layer grown between a compressive-strained quantum well and a tensile-strained GaNAs layer. Compared to a similar sample with no strain-mediating layer, this heterostructure exhibits improved material properties and remarkable redshift of emission with enhanced light intensity. The observations are based on photoluminescence spectra and x-ray diffraction data measured for the active region of the samples.

1.32-μm GaInNAs-GaAs laser with a low threshold current density

In this letter, we report on a recent development of diluted nitride laser diodes operating at the wavelengths around 1.3 /spl mu/m. The lasers grown by molecular beam epitaxy and processed into 20-/spl mu/m-wide ridge waveguide structures, mounted episides up on subcarriers, exhibit a threshold current density as low as 563 A/cm/sup 2/, slope efficiency of 0.2 W/A per facet, light power up to 40-mW continuous-wave, and characteristic temperature of 97-133 K.

Low-loss 1.3um GaInNAs saturable bragg reflector for high-power picosecond neodymium lasers

A novel low-loss, single-step-growth 1.3‐µm GaInNAs saturable Bragg reflector mode-locking element has been developed. Combined radial thickness and postgrowth annealing control have permitted a tuning range of 46 nm for passive mode locking to be demonstrated from one wafer. With this structure, stabilized mode locking was obtained from quasi-cw diode-pumped Nd:YLF and Nd:YALO lasers operating at 1314 and 1342 nm, respectively, with average on-time output powers of as much as 20 W and pulse durations as low as 22 ps.

Growth-temperature-dependent (self-)annealing-induced blueshift of photoluminescence from 1.3 μm GaInNAs/GaAs quantum wells

Growing the capping layer of a GaInNAs/GaAs quantum well at typical substrate temperature for GaAs growth by molecular-beam epitaxy, like 580 °C, was found to induce a blueshift of the quantum-well emission whose magnitude significantly increased as the quantum-well growth temperature was decreased. The growth-temperature-dependent (self-)annealing-induced blueshift is correlated with the presence of indium and occurs without observable changes in alloy macroscopic composition or quantum-well structure. The underlying cause for the increase in blue shift with decreasing quantum-well growth temperature appears to be an enhancement in the amount of In–N bonds formed by (self-)annealing, likely through a defect-assisted mechanism.

Suppression of interfacial atomic diffusion in InGaNAs/GaAs heterostructures grown by molecular-beam epitaxy

We have studied the effects of annealing of InGaNAs/GaAs heterostructures on diffusion at the interfaces and the resultant changes in optical and structural properties. Interdiffusion between In and Ga was found to be very significant. Inserting a thin compressively strained layer of InxdGa1−xdNydAs1−yd on either side of an InxqGa1−xqNyqAs1−yq quantum well (QW) suppressed this interdiffusion significantly. As a consequence, a blue shift of the photoluminescence signal after annealing remained small and the optical activity was largely improved. It was also found that a small amount of N incorporated in InGaAs QWs embedded in GaAs increased the In/Ga interdiffusion and that increased mechanical stresses enhanced the interdiffusion.

Strain-compensated GaInNAs structures for 1.3-/spl mu/m lasers

GaAs-based dilute nitride lasers are potential light sources for future optical fiber communication systems at the wavelength of 1.3 /spl mu/m. In this paper we discuss the results of studies of optimization of the growth conditions and active regions of the GaAs-based lasers. To this end, a series of samples were grown using the molecular beam epitaxy technique. The active regions consisted of quantum wells, strain-compensating layers, and strain-mediating layers. They were characterized by photoluminescence and double crystal X-ray diffraction methods. The optical properties were very much affected by a choice of growth conditions, details of the quantum wells, and postgrowth thermal treatment. Preliminary results on diode-pumped vertical-cavity surface emitting lasers, which launch light power of 3.5 mW coupled into a single-mode fiber, are also presented.