Stable semiconductor laser with a 7-Hz linewidth by an optical-electrical double-feedback technique

Robust self-injection locking to a non-confocal monolithic Fabry-Perot cavity

We demonstrate an efficient simultaneous self-injection locking of two semiconductor lasers to high-order modes of a standalone monolithic non-confocal Fabry-Perot cavity. The lasers are used to generate a low-noise microwave signal on a fast photodiode. The overall improvement of the laser spectral purity exceeds 80 dB. The observed single-sideband phase noise of X- to W-band signals is at the -110 dBc/Hz level and is limited by the fundamental thermorefractive noise of the cavity. The demonstrated cavity-laser configuration can be tightly packaged and is promising for the generation of high-frequency RF signals as well as for referencing optical frequency combs.

Semiconductor laser linewidth reduction by six orders of magnitude via delayed optical feedback

We demonstrate a coherence increase by six orders of magnitude of a standard quantum well semiconductor laser. Using a simple, optical-fiber-based feedback scheme, we stabilize the laser in a high-gain mode of a long external cavity. In a modified self-heterodyne measurement, we mix the high-gain mode with a strongly suppressed side mode and obtain an interference linewidth of only 12.6 Hz, corresponding to a decoherence of (3.1±2.9) Hz. In an independent characterization using an etalon, we deduce an upper limit of 300 Hz for the laser linewidth. The laser stably resides in this mode for tens of seconds. Our results agree with theoretical predictions.

Ultralow noise miniature external cavity semiconductor laser

Advanced applications in optical metrology demand improved lasers with high spectral purity, in form factors that are small and insensitive to environmental perturbations. While laboratory-scale lasers with extraordinarily high stability and low noise have been reported, all-integrated chip-scale devices with sub-100 Hz linewidth have not been previously demonstrated. Lasers integrated with optical microresonators as external cavities have the potential for substantial reduction of noise. However, stability and spectral purity improvements of these lasers have only been validated with rack-mounted support equipment, assembled with fibre lasers to marginally improve their noise performance. In this work we report on a realization of a heterogeneously integrated, chip-scale semiconductor laser featuring 30-Hz integral linewidth as well as sub-Hz instantaneous linewidth.

Wideband tunable laser phase noise reduction using single sideband modulation in an electro-optical feed-forward scheme

A wideband laser phase noise reduction scheme is introduced where the optical field of a laser is single sideband modulated with an electrical signal containing the discriminated phase noise of the laser. The proof-of-concept experiments on a commercially available 1549 nm distributed feedback laser show linewidth reduction from 7.5 MHz to 1.8 kHz without using large optical cavity resonators. This feed-forward scheme performs wideband phase noise cancellation independent of the light source and, as such, it is compatible with the original laser source tunability without requiring tunable optical components. By placing the proposed phase noise reduction system after a commercial tunable laser, a tunable coherent light source with kilohertz linewidth over a tuning range of 1530-1570 nm is demonstrated.

Semiconductor laser phase-noise cancellation using an electrical feed-forward scheme

We demonstrate the reduction of semiconductor laser phase noise by using an electrical feed-forward scheme. We have carried out proof-of-concept experiments on a commercially available distributed-feedback laser emitting at the 1550 nm communication band. The preliminary results show more than 20 times reduction in the phase-noise power spectrum. The feed-forward scheme does not have the limited bandwidth, stability, and speed issues that are common in feedback systems. Moreover, in the absence of electronic noise, feed-forward can completely cancel the close-in phase noise. In this scheme, the ultimate achievable phase noise will be limited by the electronics noise. Using the proposed feed-forward approach, the linewidth of semiconductor lasers can be reduced by 3-4 orders of magnitude in a monolithic approach using today's low-noise scaled transistors with terahertz gain-bandwidth product.

Compact, filtered diode laser system for precision spectroscopy

Stable, narrow-linewidth optical sources are necessary in modern atomic physics. An appealing approach to achieving approximately 10 kHz frequency stability is optical feedback. We have designed a compact external cavity diode laser with optical feedback to a filter cavity mounted on a single baseplate and enclosed inside a vacuum sealed box. The design was implemented for three wavelengths addressing the 422 nm cooling, 1091 nm repumping, and 674 nm clock transition lines of Sr(+). We are able to cool a single, trapped strontium ion to approximately 2 mK and observe motional sidebands of the 5S(1/2) <--> 4D(5/2) transition.

Linewidth reduction by 6 orders of magnitude of a broad-area 729-nm diode laser

Diode lasers with a power output superior to 100 mW are in widespread use in medical as well as research applications. However, for such diodes lasing oscillation generally occurs simultaneously in several longitudinal and transverse modes that are unsuitable for high-resolution spectroscopy. We spectrally narrow a 100-mW broad-area diode laser by first using an extended cavity and then an electrical feedback produced by a Pound-Drever-Hall stabilization on a low-finesse reference cavity. Reduction of the linewidth by more than 6 orders of magnitude is achieved (the output linewidth is narrowed from 1 THz to less than 500 kHz), making possible its use for high-resolution spectroscopy. The power and the spectral qualities of this diode laser allow us to induce quantum jumps toward the D5/2 metastable level of single Ca+ ions.

Frequency noise reduction in erbium-doped fiber distributed-feedback lasers by electronic feedback

A novel technique for suppressing frequency noise in an erbium-doped fiber distributed-feedback laser incorporated into a master-oscillator-power-amplifier configuration by an electronic feedback technique is presented. The frequency noise is suppressed by locking of the laser emission to a fiber interferometer. The frequency noise spectral density of the laser is reduced by as much as 20 dB over the frequency range 1 Hz-10 kHz to 1.5 Hz/Hz(1/2) +/-25% at 1 kHz with a relative intensity noise spectral density below -120 dB/Hz over the frequency range 10 Hz-1 kHz. These lasers will have applications as sources for fiber-optic interferometry, high-resolution spectroscopy, and high-bandwidth communications.

193-mHz beat linewidth of frequency-stabilized laser-diode-pumped Nd:YAG ring lasers

We describe frequency-stabilized laser-diode-pumped Nd:YAG ring lasers locked to a high-transmission and high-finesse reference Fabry-Perot cavity. The cavity has a resonant linewidth of 38.0 kHz, a finesse of 19,400, and a transmission efficiency of 79.4% for the TEM(00) mode. We measure the frequency stability by locking two lasers one free spectral range apart and observe the heterodyne beat signal. As a result, the spectral densities of frequency noise are reduced to 10 mHz/ radicalHz, and a beat linewidth of 193 mHz is obtained. The linewidth is 0.73 times the Schawlow-Townes limit.

Frequency noise reduction of a diode laser by using the FM sideband technique

The frequency of a 1.5-microm three-electrode multiple-quantum-well distributed-feedback diode laser was stabilized to a reference Fabry-Perot cavity by using a current-modulated FM sideband technique. The low-frequency AM noise imposed on the frequency error signal was effectively reduced with this technique, and the power spectrum density of the FM noise was reduced to less than 25 Hz(2) /Hz at a Fourier frequency of less than 10 kHz, which corresponded to a Lorentz spectral half-linewidth of approximately 80 Hz.

Frequency stabilization of a semiconductor laser using an external phase modulator

A Ti:LiNbO(3) waveguide phase modulator is used as an external frequency stabilizer to reduce the linewidth of an AlGaAs semiconductor laser. A ring interferometer is used as a frequency discriminator, and a wideband servo controller (dc to 30 MHz) is constructed in which feed-forward control of the external phase modulator is combined with feedback control of the injection current. A linewidth of 45 kHz is obtained.

Injection locking of a highly coherent and high-power diode laser at 1.5 microm

Injection locking has been employed to improve the coherence of a high-power diode laser at 1.5 microm. The injected high-power laser emitted in a single mode with a side-mode suppression ratio of larger than 30 dB and an output power of 40 mW. The FM noise of the slave laser was nearly the same as that of the submegahertz-linewidth master laser. We have also demonstrated the coherent addition of the master and the injection-locked slave laser with a residual phase error of deltaø < 0.2 rad.