S. Schilt

Frequency noise of free-running 4.6 μm distributed feedback quantum cascade lasers near room temperature

The frequency noise properties of commercial distributed feedback quantum cascade lasers emitting in the 4.6 μm range and operated in cw mode near room temperature (277 K) are presented. The measured frequency noise power spectral density reveals a flicker noise dropping down to the very low level of <100 Hz(2)/Hz at 10 MHz Fourier frequency and is globally a factor of 100 lower than data recently reported for a similar laser operated at cryogenic temperature. This makes our laser a good candidate for the realization of a mid-IR ultranarrow linewidth reference.

Rb-stabilized laser at 1572 nm for CO2 monitoring

We have developed a compact rubidium-stabilized laser system to serve as optical frequency reference in the 1.55-μm wavelength region, in particular for CO2 monitoring at 1572 nm. The light of a fiber-pigtailed distributed feedback (DFB) laser emitting at 1560 nm is frequency-doubled and locked to a sub-Doppler rubidium transition at 780 nm using a 2-cm long vapor glass cell. Part of the DFB laser light is modulated with an electro-optical modulator enclosed in a Fabry-Perot cavity, generating an optical frequency comb with spectral coverage extending from 1540 nm to 1580 nm. A second slave DFB laser emitting at 1572 nm and offset-locked to one line of the frequency comb shows a relative frequency stability of 1.10-11 at 1 s averaging time and <4.10-12 from 1 hour up to 3 days.

Physical origin of frequency noise and linewidth in mid-IR DFB quantum cascade lasers

Frequency noise and linewidth properties of different mid-infrared DFB-QCLs using buried-heterostructures and ridge waveguides are compared. The physical origin of frequency noise and the impact of the different lasers parameters are discussed.

Frequency noise of free-running room temperature quantum cascade lasers

Narrow-linewidth lasers exhibiting a high spectral purity have important applications in various fields, such as high-resolution spectroscopy, coherent optical communications or time and frequency metrology, to name a few. With a proper feedback loop for linewidth reduction, such lasers can be used as ultra-stable reference for low phase noise microwave generation in frequency metrology. Since mid-infrared quantum cascade lasers (QCLs) are expected to have a narrow intrinsic linewidth (due to a linewidth-enhancement factor αH close to 0), we have recently investigated the frequency noise properties of commercial free-running singlemode QCLs emitting in the 4.6-µm wavelength range and operated in CW mode near room temperature.

1.5-µm Cavity-stabilized laser for ultra-stable microwave generation

Based on an extended-cavity diode laser locked to a high-finesse ultra-low thermal expansion Fabry-Perot cavity using the Pound-Drever-Hall (PDH) technique, we are developing an ultra-stable laser at 1.55 μm for the generation of low phase noise microwave. Our target short-term frequency instability is 10−15 at 1 s. The horizontally-mounted cavity has a resonance width of 8.1 kHz, experimentally determined from the cavity ring-down time. The analysis of the in-loop error signal of the laser stabilization to the cavity shows that the residual servo loop noise contributes only <10−15 (@ 1 s) to the laser instability and no additional contribution to the laser linewidth is induced by the PDH stabilization. Consequently, the laser linewidth will be ultimately limited by the cavity noise only. The cavity will be stabilized at its zero coefficient of thermal expansion temperature, which has been experimentally determined to be 22.6°C.

Frequency comb metrology with an optical parametric oscillator

We report on the first demonstration of absolute frequency comb metrology with an optical parametric oscillator (OPO) frequency comb. The synchronously-pumped OPO operated in the 1.5-µm spectral region and was referenced to an H-maser atomic clock. Using different techniques, we thoroughly characterized the frequency noise power spectral density (PSD) of the repetition rate frep, of the carrier-envelope offset frequency fCEO, and of an optical comb line νN. The comb mode optical linewidth at 1557 nm was determined to be ~70 kHz for an observation time of 1 s from the measured frequency noise PSD, and was limited by the stability of the microwave frequency standard available for the stabilization of the comb repetition rate. We achieved a tight lock of the carrier envelope offset frequency with only ~300 mrad residual integrated phase noise, which makes its contribution to the optical linewidth negligible. The OPO comb was used to measure the absolute optical frequency of a near-infrared laser whose second-harmonic component was locked to the F = 2→3 transition of the 87Rb D2 line at 780 nm, leading to a measured transition frequency of νRb = 384,228,115,346 ± 16 kHz. We performed the same measurement with a commercial fiber-laser comb operating in the 1.5-µm region. Both the OPO comb and the commercial fiber comb achieved similar performance. The measurement accuracy was limited by interferometric noise in the fibered setup of the Rb-stabilized laser.

Opto-Optical Modulation of an Intra-Cavity SESAM for Low-Noise CEO Stabilization of a Femtosecond Laser

We present a CEO-stabilization technique based on optical feedback to an intra-cavity SESAM with significantly improved bandwidth compared to standard pump current control, enabling a CEO-locked Er:Yb:glass laser with ten times lower residual phase noise.

First fully stabilized frequency comb from a SESAM-modelocked 1.5-µm solid-state oscillator

In conclusion, we have demonstrated the first fully stabilized frequency comb generated by an ultrafast Er:Vb:glass laser. The SESAM-modelocked, diode-pumped laser uses a high-Q cavity which results in low-noise operation. A coherent, octave-spanning SC is generated by nonlinear spectral broadening in a polarization maintaining highlynonlinear fiber. We observe a more than 10-times improvement in the free-running CEO frequency linewidth compared to free-running femtosecond fiber laser systems operating in this spectral region. The CEO frequency and the repetition rate are stabilized to the same 10-MHz external reference over hours. The Allan deviation of the 20-MHz CEO reaches 10-8 at 1-s with an H-maser as an external reference and a stable locked operation of more than 11 hours was achieved in a preliminary long-term evaluation. A CEO-beat signal can still be observed at a large reduction of the pump power, resulting in a reduced intracavity pulse energy and in an increased pulse duration up to 260 fs. We expect that this result will have a significant impact for the future development of more compact stable frequency combs as the relatively long pulse duration relaxes the requirements on the modelocked laser. This will become even more important for gigahertz pulse repetition rates.

Optical frequency comb stabilization of a gigahertz semiconductor disk laser

Semiconductor lasers are a promising technology to make optical comb systems more accessible and cost-efficient. We stabilized the carrier-envelope offset (CEO) frequency of a semiconductor disk laser. The laser was modelocked by a SESAM and generates pulses at a wavelength of 1034 nm. It operates at a repetition frequency of 1.8 GHz. The 270-fs pulses are amplified to 3 W and compressed to 120 fs. A coherent octave-spanning supercontinuum spectrum is generated in a highly nonlinear fiber. Using a standard f-to-2f interferometer, we detect the CEO beat with a signal-to-noise ratio of ~30 dB. By applying a feedback signal to the pump current, the CEO frequency is phase-locked to an external reference.

Revisiting the relation between laser spectrum and phase noise spectral density with new outcomes

We revisit an expansion series introduced a long time ago by D. Middleton to link the phase noise power spectral density (PN-PSD) of an oscillator to its power spectrum, which is poorly known and used in the laser community. In contrast, a different approach proposed by D. Elliott is widely used to compute the spectrum of a laser from its PN-PSD. We show some benefits brought by the use of Middletons series both on the conceptual and quantitative points of view and we introduce a simple guideline to implement this approach with an arbitrary PN-PSD. We illustrate these considerations with several experimental examples.