Frequency-locked tunable LD laser with a broad bandwidth

Using lock-in amplifer and proportional, integral, and derivative (PID) electric circuit, the frequency of diode laser is stabilized on a highly mechanical stable Fabry-Perot (FP) cavity transmission peak. When the frequency locking system is on, the frequency tunable range of the laser is about 4 GHz around the D1 transition of Rb. The laser frequency tuning is implemented by scanning the FP cavity length. The fluctuation of frequency of the output laser is less than 1 MHz, and the drift of the center frequency is less than 1.5 MHz in 1.5 min. This system has great potential of the application in the experimental investigation of the interaction between light and atoms, especially, for the case of far off the atomic resonance.     

Improving Locking Accuracy of Resonant Optical Gyroscope by Laser and Acoustooptic Frequency Shifter Jointed Pound-Drever-Hall Technique

ABSTRACT

A laser and acoustooptic frequency shifter jointed Pound-Drever-Hall technique is designed to lock the resonant optical gyroscope. Utilizing direct digital synthesizer (DDS) technology and temperature control in the driver, the frequency-shift precision of the acoustooptic frequency shifter higher than 1 Hz is obtained. Experiments show that compared with locking by tuning the laser only, the zero-bias stability of the gyroscope is improved. In this locking scheme, there are few additional devices, the tuning of the laser and the acoustooptic frequency shifter share a common Pound-Drever-Hall locking system, and the demodulation and the locking algorithms are simple.     

Laser frequency offset locking scheme for high-field imaging of cold atoms

We present a simple and flexible frequency offset locking scheme developed for high-field imaging of ultra-cold atoms which relies on commercially available RF electronics only. The main new ingredient is the use of the sharp amplitude response of a “home-made” RF filter to provide an error signal for locking the lasers. We were able to offset lock two independent diode lasers within a capture range of hundreds of MHz, and with a tuning range of up to 1.4 GHz. The beat-note residual fluctuations for offset locked lasers are below 2 MHz for integration times of several hundreds of seconds.     

A high-energy, chirped laser system for optical Stark deceleration

We describe a high-energy, frequency chirped laser system designed for optical Stark deceleration of cold molecules. This system produces two, pulse amplified beams of up to 700 mJ with flat-top temporal profiles, whose frequency and intensity can be well controlled for durations from 20 ns–10 μs. The two beams are created by amplifying a single, rapidly tunable Nd:YVO₄ microchip type laser at 1064 nm, which can be frequency chirped by up to 1 GHz over the duration of the pulse. Intensity modulation induced by relaxation oscillations in the microchip laser during the frequency chirp are virtually eliminated by injection locking a free running semiconductor diode laser before pulsed amplification.     

Wavelength swept Ti:sapphire laser

In this study, we demonstrate, for the first time to our knowledge, electronic wavelength sweeping of a continuous wave Ti:sapphire laser using an acousto-optic tunable filter (AOTF). The dependence of the laser output on the sweeping frequency and on the spectral tuning range was investigated. The lasing up to maximum scan rate 11 kHz for 10 nm tuning range and 5 W pump power was achieved. We detected and quantified asymmetry in the output for opposite scan directions. We theoretically characterized the maximum sweeping frequency for swept lasers with AOTFs and confirmed calculated results by measurements.     

Seed injection and frequency-locked Nd:YAG laser for direct detection wind lidar

The single longitudinal mode operation and frequency stability are essential for the laser transmitter used in the Doppler lidar. We devise a seed injection, frequency tunable and locked Q-switched Nd:YAG laser for the direct detection Doppler lidar. By implementation of the dual-wavelength seed laser and iodine-based PID frequency-locking technique, the frequency-stabilized seed laser is robust to interference and can be locked within 200 kHz for 3 h. The stable output of single longitudinal mode of the frequency-doubled pulsed laser makes it possible to achieve operational wind measurement.     

Injection locking of a diode laser locked to a Zeeman frequency stabilized laser oscillator

We describe the design and performance of an injection-locked diode laser locked to a stabilized, single frequency, unmodulated diode laser. The master oscillator is a grating-tuned, external cavity diode laser which is stabilized on a Doppler free alkali metal resonance transition frequency via Zeeman locking. The master oscillator frequency is shifted by an acousto-optic modulator, which provides optical isolation of the master oscillator laser while tuning of the acousto-optic modulation frequency can also provide frequency offset tuning. The slave laser is a free running diode which is injection-locked by a small fraction of the frequency shifted master oscillator light. Good long- and short-time frequency stability are observed for both the Zeeman-locked master oscillator and the injection-locked slave laser.     

Dispersion-tuned multiwavelength actively mode-locked fiber laser using a hybrid gain medium

We demonstrate a multiwavelength 10 GHz pulse source using a dispersion-tuned actively mode-locked fiber ring laser incorporated with a semiconductor optical amplifier and an erbium-doped fiber amplifier. Simultaneous seven-wavelength operation of the laser is obtained. The side-mode suppression of all wavelengths is above 30 dB. Smooth wavelength tuning is achieved over more than 12 nm by changing the modulation frequency or the length of the optical delay line. Pulse characteristics are almost constant over the entire tuning span. Wavelength spacing can also be varied from 0.9 to 10 nm by adjusting the dispersion of the cavity. These experimental observations agree well with theoretical analyses.     

Research on tunable local laser used in ground-to-satellite coherent laser communication

In order to realize homodyne reception and Doppler frequency shift tracking in ground-to-satellite coherent laser communication, a local laser is experimentally demonstrated in this Letter. It is realized based on modulationsideband injection locking, and has a 10 GHz tuning range, a 1 THz/s tuning rate, a 5 k Hz linewidth, and 16 m W of output power. When applied to a Costas loop in a coherent laser communication system, the local laser can achieve ?5 GHz Doppler frequency shift tracking with a 20 MHz/s frequency shift rate, which is sufficient for the ground-to-satellite coherent laser communication.     

Offset frequency locking for nanosecond laser pulses

A method of multi-pulse discriminating frequency and high probability average value filtering is presented for offset frequency locking with ns laser pulses. In the experiment, the frequency locking for cavity-dumped CO₂ laser with 100 ns pulse width and the repetition rate of 10 kHz was studied. The precision was up to ±2 MHz at the heterodyne frequency 90 MHz. However, it is more than ±10 MHz for the single pulse discriminating frequency. This method can also be applied to laser offset frequency locking for many kinds of short pulse lasers.     

Theoretical analysis of a quartz-enhanced photoacoustic spectroscopy sensor

Quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors are based on a recent approach to photoacoustic detection which employs a quartz tuning fork as an acoustic transducer. These sensors enable detection of trace gases for air quality monitoring, industrial process control, and medical diagnostics. To detect a trace gas, modulated laser radiation is directed between the tines of a tuning fork. The optical energy absorbed by the gas results in a periodic thermal expansion which gives rise to a weak acoustic pressure wave. This pressure wave excites a resonant vibration of the tuning fork thereby generating an electrical signal via the piezoelectric effect. This paper describes a theoretical model of a QEPAS sensor. By deriving analytical solutions for the partial differential equations in the model, we obtain a formula for the piezoelectric current in terms of the optical, mechanical, and electrical parameters of the system. We use the model to calculate the optimal position of the laser beam with respect to the tuning fork and the phase of the piezoelectric current. We also show that a QEPAS transducer with a particular 32.8 kHz tuning fork is 2–3 times as sensitive as one with a 4.25 kHz tuning fork. These simulation results closely match experimental data.     

Phase and spectral properties of optically injected semiconductor lasers

The main control parameters of a single mode semiconductor laser submitted to an injected external signal are the power and the frequency of the injected signal. Following their magnitude, many phenomena can be observed such as phase locking, frequency locking, frequency generation, push-pull effects, hysteresis phenomena and chaos,... We show here that the spectral signature of the slave laser enables a better understanding of the the nonlinear interaction between the two competing sources: the spontaneous emission and the external field for which spectra are equally amplified through the active medium. This amplification is then strongly dependent on their coherency. We describe the role of the injected laser as a filter and an amplifier. It follows that the laser can be used to process information in ways that are not yet completely exploited. To cite this article: S. Blin et al., C. R. Physique 4 (2003).     

Widely tunable THz synthesizer

The generation of cw-THz radiation by photomixing is particularly suited to the high resolution spectroscopy of gases; nevertheless, until recently, it has suffered from a lack of frequency metrology. Frequency combs are a powerful tool that can transfer microwave frequency standards to optical frequencies and a single comb has permitted accurate (10⁻⁸) THz frequency synthesis with a limited tuning range. A THz synthesizer composed of three extended cavity laser diodes phase locked to a frequency comb has been constructed and its utility for high resolution gas phase spectroscopy demonstrated. The third laser diode allows a larger tuning range of up to 300 MHz to be achieved without the need for large frequency excursions, while the frequency comb provides a versatile link to be established from any traceable microwave frequency standard. The use of a single frequency comb as a reference for all of the cw-lasers eliminates the dependency of synthesized frequency on the carrier envelope offset frequency. This greatly simplifies the frequency comb stabilization requirements and leads to a reduced instrument complexity.     

Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking

A new technique of cavity enhanced absorption spectroscopy is described. Molecular absorption spectra are obtained by recording the transmission maxima of the successive TEM_oo resonances of a high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them. A noisy cavity output is usually observed in such a measurement since the resonances are spectrally narrower than the laser. We show that a folded (V-shaped) cavity can be used to obtain selective optical feedback from the intracavity field which builds up at resonance. This induces laser linewidth reduction and frequency locking. The linewidth narrowing eliminates the noisy cavity output, and allows measuring the maximum mode transmissions accurately. The frequency locking permits the laser to scan stepwise through the successive cavity modes. Frequency tuning is thus tightly optimized for cavity mode injection. Our setup for this technique of Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS) includes a 50 cm folded cavity with finesse ∼20 000 (ringdown time ∼20 μs) and allows recording spectra of up to 200 cavity modes (2 cm⁻¹) using 100 ms laser scans. We obtain a noise equivalent absorption coefficient of ∼5×10⁻¹⁰ cm⁻¹ for 1 s averaging over scans, with a dynamic range of four orders of magnitude.     

Mode competition and frequency locking

A theoretical study is presented of frequency locking to be observed in lasers with broad homogeneous emission lines, when the cavity losses are reduced, at a certain frequency ω₀, by placing inside the cavity a suitable element (e.g., a selectively reflecting “mirror”). Two kinds of locking effects are analyzed: i) When tuning the center frequency of the laser output ω_f towards ω₀, by means of a filter or a grating, the laser frequency becomes locked to ω₀, when |ω_f ? ω₀| reaches a critical value. ii) In the absence of frequency-selective elements, the laser oscillates only in a narrow spectral range centered at ω₀. Special emphasis is given to the role played by mode competition in these locking phenomena.     

Frequency comb-referenced narrow line width diode laser system for coherent molecular spectroscopy

We analyze in detail the frequency noise properties of a grating enhanced external cavity diode laser (GEECDL). This system merges two diode laser concepts, the grating stabilized diode laser and the diode laser with resonant optical feedback, thus combining a large tuning range with an excellent short-term frequency stability. We compare the frequency noise spectrum of a GEECDL to that of a grating stabilized diode laser and demonstrate a 10-fold reduction of the frequency noise linear spectral density. The GEECDL is phase locked to a similar laser and to a fs-frequency comb with a servo loop providing an open-loop unity-gain frequency of only 237 kHz, which is a tenth of the bandwidth typically required for grating stabilized diode lasers. We achieve a residual rms phase error as small as 72 mrad (≈ 200 mrad) for stabilization to a similar laser (to the fs-frequency comb). We demonstrate that the novel diode laser can phase-coherently track a stable optical reference with an instability of 1.8×10^-16 at 1 s. This laser system is well suited for applications that require phase locking to a low-power optical reference under noisy conditions. It may also be considered for the implementation of optical clock lasers. PACS 42.55.Px; 42.60.Jf; 42.50.Gy     

C-band continuously tunable lasers using tunable fiber Bragg gratings

We report the development of a ring tunable fiber laser based on tunable fiber Bragg gratings (TFBG) integrated with an optical circulator. The TFBG is embedded inside a 3-piont bending device for wavelength tuning. The tunable laser operating in the C-band has power variation, tuning resolution, tuning range and laser line width of ±0.5 dB, 0.5 nm, 25.0 nm and less than 0.05 nm, respectively. As 40 mW of pump power is used, the ring tunable laser has a side mode suppression ratio of 60 dB and a power conversion efficiency of 25%. These specifications ensure the high-quality operation of a tunable laser.     

Fine frequency tuning in sum-frequency generation of continuous-wave single-frequency coherent light at 252 nm with dual-wavelength enhancement

Fine frequency tuning of the deep-ultraviolet single-mode coherent light at 252 nm was conducted through the PID feedback system automatically by changing the temperature of a beta-BaB(2)O(4) (BBO) crystal in a doubly resonant external cavity for the sum-frequency mixing of 373 and 780 nm light. The temperature-dependent frequency tuning rate is 19.3 MHzK(-1), which is sufficiently fine to realize the laser cooling of neutral silicon atoms because the natural width of the laser cooling transition is 28.8 MHz.     

Versatile Raman fiber laser for sodium laser guide star

Robust high‐power narrow‐linewidth lasers at 589 nm are required for sodium laser guide star adaptive optics in astronomy. A high‐power 589 nm laser based on Raman fiber amplifier is reported here, which works in both continuous‐wave and pulsed formats. In the continuous‐wave case, the laser produces more than 50 W output. In the pulsed case, the same laser produces square‐shaped pulses with tunable repetition rate (500 Hz to 10 kHz) and duration (1 ms to 30 μs). The peak power is as high as 84 W and remains constant during the tuning. The laser also emits an adjustable sideband at 1.71 GHz away from the main laser frequency for better sodium excitation. The versatility of the laser offers much flexibility in laser guide star application.