Extended-cavity Semiconductor Laser Research Articles

Extended ultrahigh-Q-cavity diode laser.

We report on a study of a 698 nm extended cavity semiconductor laser with intracavity narrowband optical feedback from a whispering gallery mode resonator. This laser comprises an ultrahigh-Q (>10(10)) resonator supporting stimulated Rayleigh scattering, a diffraction grating wavelength preselector, and a reflective semiconductor amplifier. Single longitudinal mode lasing is characterized with sub-kilohertz linewidth and a 9 nm coarse tuning range. The laser has a potential application for integration with the 1S0-3P0 strontium transition to create compact precision atomic clocks.

EIT as a tool to observe dual wavelength operation of a semiconductor laser

We propose a simplified technique for dual wavelength operation of an extended cavity semiconductor laser, and its characterization using electromagnetically induced transparency (EIT). In this laser cavity scheme light beam is made converging before it incidences on the cavity grating. The converging angle of the beam creates two longitudinal oscillating modes of resonating cavity. Frequency separation between the longitudinal modes are measured with the help of beat frequency generation in a photodiode and creating pair of EIT spectra in Rb vapor. The pair of EIT dips that are generated due to dual wavelength of this laser (that is used as control laser) can be used to estimate frequency difference between the generated wavelengths. Width of EIT spectra can be used to estimate linewidth of individual wavelength components.

High power single–frequency continuously–tunable compact extended–cavity semiconductor laser

We demonstrate high power high efficiency (0:3 W) low noise single frequency operation of a compact extended-cavity surface-emitting-semiconductor-laser exhibiting a continuous tunability over 0:84 THz with high beam quality. We took advantage of thermal lens-based stability to develop a short (< 3 mm) plano-plano external cavity without any intracavity filter. The structure is optically pumped by a 1 W commercial 830 nm multimode diode laser. No heat management was required. We measured a low divergence circular TEM(00) beam at the diffraction limit (M(2) < 1:05) with a linear light polarization (> 37 dB). The side mode suppression ratio is 60 dB. The free running laser linewidth is 850 kHz limited by pump induced thermal fluctuations. Thanks to this high-Q external cavity approach, the frequency noise is low and the dynamics is in the relaxation-oscillation-free regime, exhibiting a low intensity noise, with a cutoff frequency approximately 250 MHz above which the shot noise level is reached. We show that pump properties define the cavity design and laser coherence.

Modal competition via four-wave mixing in single-mode extended-cavity semiconductor lasers

An analysis of the wave-coupling phenomena in extended-cavity semiconductor lasers is carried out to investigate the conditions of mode-hop-free single-mode operation. Taking into account the external cavity selectivity and the mode coupling via four-wave mixing in the active medium, we present a theoretical analysis of the modal competition and perform calculations that are in good agreement with experimental results obtained with a 1.55-/spl mu/m extended-cavity laser.

Relaxation of the single-mode emission conditions in extended-cavity semiconductor lasers with a self-organizing photorefractive filter.

Commercial 1.55-microm extended-cavity semiconductor lasers provide single-mode operation that can be continuously tuned over a range larger than 100 nm without mode hopping. But such performance requires delicate factory adjustment and high mechanical stability of the external cavity. Furthermore, at high emission power the tuning range is limited to small values because of the annoying multimode operations that sometimes occur. We have shown that the alignment constraints can be relaxed by use of an intracavity photorefractive filter. Here we present new results obtained with a crystal with low absorption and high photorefractive gain. We demonstrate that, without inducing excessive additional loss, we can preserve single-mode emission at an output power higher than the maximum power obtained in the absence of a photorefractive crystal over the full tuning range of the laser.

The design of a compact and tunable extended-cavity semiconductor laser

The design of a tunable extended-cavity semiconductor laser based on the modifiedLittman configuration is described. The mechanics of the extended cavity is designed withrespect to applications as a stabilized laser source for an etalon of optical frequency, a laserfor interferometry where stability, narrow linewidth, and mode hop free tuningrange are required and for spectroscopic purposes with similar requirements. Itfollows the contradicting demands for a rigid construction and fine adjustabilityof the optical components. Optimisation of the optical feedback level and itsinfluence of the laser behaviour are discussed and a method for the feedback leveladjustment by rotation of the polarization plane is proposed. The laser parameters arepresented for a version based on a laser diode operating at a wavelength of 633 nm.

High-speed wavelength-swept semiconductor laser with a polygon-scanner-based wavelength filter.

Ultrahigh-speed tuning of an extended-cavity semiconductor laser is demonstrated. The laser resonator comprises a unidirectional fiber-optic ring, a semiconductor optical amplifier as the gain medium, and a novel scanning filter based on a polygonal scanner. Variable tuning rates up to 1150 nm/ms (15.7-kHz repetition frequency) are demonstrated over a 70-nm wavelength span centered at 1.32 microm. This tuning rate is more than an order of magnitude faster than previously demonstrated and is facilitated in part by self-frequency shifting in the semiconductor optical amplifier. The instantaneous linewidth of the source is <0.1 nm for 9-mW cw output power and a low spontaneous-emission background of -80 dB.

Compact solid-state waveguide lasers

Glass waveguide lasers will fill an important niche as optical sources in communication, RF photonics, and optical metrology. This is primarily because waveguide lasers benefit from compact size, low noise, relatively high output powers, long upper-state lifetimes, and simple integration with optical-fiber-based systems. Although we do not expect waveguide lasers and amplifiers to ever supplant fiber and semiconductor lasers and amplifiers in every possible communications application, waveguide lasers have a number of advantages over traditional lasers for these uses. Single-frequency waveguide lasers provide narrow linewidth and high output power in a compact, monolithic package. The narrow linewidth is an important advantage over standard semiconductor lasers, and the compact size makes single-frequency waveguide lasers better suited than fiber lasers or extended-cavity semiconductor lasers for many applications.

Stabilization of a 1.55 μm extended-cavity semiconductor laser by intracavity dynamic holography

Commercial grating-tuned single-mode extended-cavity semiconductor lasers (ECLD's) can be tuned over 100 nm around 1.55 μ m. This continuous tuning with no mode-hopping requires delicate factory adjustments and a high mechanical stability. These constrains are relaxed by the adjunction of a photorefractive crystal inside the cavity which creates an adaptive spectral filter which decreases the loss of the lasing-mode and thus enhances its stability.

Side-mode gain in grating-tuned extended-cavity semiconductor lasers: investigation of stable single-mode operation conditions

In semiconductor lasers, nonlinear phenomena inside the active medium change the side-mode gain with respect to the static threshold-gain. For a given side mode, the sign and the magnitude of the change depend on the lasing-mode optical power and on the wavelength detuning from the lasing mode. If the side-mode gain is enhanced, mode-hopping can occur toward a side mode whose loss is higher than the lasing-mode one. Conversely, in the case of side-mode-gain suppression, the side-mode loss can be smaller with no mode-hop. In this paper, effects of carrier-density pulsation, carrier heating, and spectral-hole burning on the conditions of stable single-mode operation in grating-tuned single-mode extended-cavity semiconductor lasers are investigated. Taking into account the external cavity spectral selectivity and mode coupling, we present a theoretical analysis of experimental results. We perform calculations that compare well with the experimental data obtained with a 1.55-/spl mu/m extended-cavity laser.

Relaxation of the alignment tolerances of a 155-μm extended-cavity semiconductor laser by use of an intracavity photorefractive filter

Commercial grating-tuned single-mode extended-cavity semiconductor lasers (ECLDs) can be tuned over 100 nm near 1.55mum . This continuous tuning with no mode hopping requires delicate factory adjustments and high mechanical stability so that the wavelength precision is kept as high as possible and the mismatch between the lasing wavelength and the wavelength of minimum loss remains as small as possible. The addition of a photorefractive crystal inside the cavity creates an adaptive spectral filter that decreases the loss of the lasing mode and thus enhances its stability. For what is to our knowledge the first time, we demonstrate the extension of the available wavelength-mismatch range without mode hopping by the addition of a CdTe photorefractive crystal inside the cavity of a single-mode grating-tuned ECLD.

Theory of large-signal direct modulation of extended cavity semiconductor lasers with dispersive loss

We present a model of coupled semiconductor diode-dispersive extended cavity lasers that describes arbitrarily large current modulation, does not assume uniform photon and carrier densities within the semiconductor diode gain medium, and is simple to implement numerically. In particular, we look at the fiber grating laser, an example of such a coupled system that has been studied as a possible optical communications source. We find simple expressions for the instantaneous frequency and intensity in terms of a few parameters characterizing the dispersion, allowing for the design of the system to minimize chirp. From these expressions we also recover Kazarinov and Henry's adiabatic chirp formula in the limit of slow modulation and Petermann's dynamical chirp formula in the limit of uniform carrier and photon densities. We show that in typical cases, where nonuniformity is important, the instantaneous frequency modulation is quantitatively and qualitatively different than is predicted by previous models.

Tunable extended-cavity diode laser stabilized on iodine at λ = 633 nm

We present a tunable extended-cavity semiconductor laser, based on the Littman configuration, which has been frequency-stabilized to Doppler-free hyperfine transitions in I2. The stability was measured compared with the reference He-Ne-I2 laser system, whereas the semiconductor laser was locked on components of the P(33) 6-3 transition close enough to the reference R(127) 11-5 line to allow beat frequency counting. A relative stability of 4 x 10(-12) over a 100-s integration time was achieved. The laser configuration allowed mode-hop-free tuning over a range including the P(33) 6-3 transition and the group of strong overlapping transitions R(60) 8-4, R(125) 9-4, and P(54) 8-4 with higher signal-to-noise ratio than the P(33) 6-3 located approximately 13 GHz toward lower optical frequencies.

Generation of millimeter-wave signals up to 70.5 GHz by heterodyning of two extended-cavity semiconductor lasers with an intracavity electro-optic crystal

Using an optical phase-locked loop (OPLL), millimeter-wave signals were generated at frequencies up to 70.5 GHz by heterodyning the outputs of two extended-cavity semiconductor lasers. One of the lasers had an intracavity electro-optic crystal to control the lasing frequency. The FM-response characteristics of the laser were examined and improved by giving a mechanical constraint to the crystal. For the generation of millimeter-wave signals, a portable spectrum analyzer was employed as a replacement for a local oscillator and a mixer. This implementation permitted a wide-range tuning operation from 5 GHz to 50 GHz without any alterations to the setup. The continuous tuning operation of over 150 MHz could also be achieved at every frequency. With the aid of an external harmonic mixer, the millimeter-wave frequency was extended up to 70.5 GHz.

Noise reduction in a mode-locked semiconductor laser by coherent photon seeding

We have used coherent photon seeding to reduce the noise in an actively mode-locked extended-cavity semiconductor laser. Feedback is provided by an auxiliary cavity terminated either by a wedge plate or by a phase-conjugate mirror. Both conventional and phase-conjugate feedback can reduce the low-frequency noise components (0-5 MHz) by as much as 36 dB, hence drastically reducing the pulse fluctuations. A noise reduction of as much as 18 dB is also observed in the 0.01-3.0-GHz frequency range. We obtained these results while preserving the pulse duration (26 ps) and improving the time-bandwidth product.

Improvements in mode-locked semiconductor diode lasers using monolithically integrated passive waveguides made by quantum-well intermixing

By using the technique of quantum-well intermixing (QWI), monolithically integrated passive, and active waveguides can be fabricated. It is shown that mode-locked extended cavity semiconductor lasers with integrated low-loss passive waveguides display superior performance to devices in which the entire waveguide is active: the threshold current is a factor of 3-5 lower, the pulsewidth is reduced from 10.2 ps in the all active laser to 3.5 ps in the extended cavity device and there is a decrease in the free-running jitter level from 15 to 6 ps (10 kHz-10 MHz).

Injection locking of an actively mode-locked semiconductor laser

We report what is to our knowledge the first observation of synchronous coherent injection locking of an actively mode-locked extended cavity semiconductor laser to an external light pulse train. In the temporal domain, we observed experimentally and verified theoretically a significant narrowing of the output pulse of the mode-locked laser. In the spectral domain, we demonstrated experimentally the narrowing of the output optical spectrum that was due to rejection of spectral components outside the spectral band of the locking pulses. Locking ranges, in the time and the frequency domains, were measured to be ~200 ps and ~5 nm, respectively.

Discontinuous modulation response of grating-assisted extended-cavity semiconductor lasers

The resonant enhancement of the small-signal intensity modulation response of a grating-assisted extended-cavity semiconductor laser is shown to occur in dual peaks centred near the harmonics of the cavity resonance fluency. Each response peak is bifurcated into two closely spaced peaks of different amplitudes and bandwidths. The position of the broader of the two peaks depends on the operating power of the laser. A strong discontinuity in the response amplitude is observed near the harmonics of the cavity resonance fluency.

InGaAsP monolithic extended-cavity lasers with integrated saturable absorbers for active, passive, and hybrid mode locking at 8.6 GHz

Active, passive, and hybrid mode locking of a monolithic extended-cavity semiconductor laser with an integrated saturable absorber have been demonstrated. Actively mode locked at a repetition rate of 8.6 GHz, 6.2 ps pulses were measured. Hybrid mode locking resulted in 4.4 ps pulses. The extended-cavity laser also exhibits self-starting passive mode locking at a repetition rate of 8.57 GHz with 5.5 ps pulses. A broad spectrum allows wavelength selectivity over a range of 15 nm with little change in the pulse width.

Pure phase-modulation mode locking in semiconductor lasers

Pure phase-modulation mode locking of an extended-cavity semiconductor laser, operating near 1.5 microm, is demonstrated. The mode-locked laser uses a two-section optical amplifier that provides both gain and phase modulation.