Frequency Stabilization Technique Research Articles

Quantum cascade lasers for high-resolution spectroscopy

Despite the growing interest that quantum cascade lasers (QCLs) are gaining, they still present a few unclear aspects of their fundamental properties, such as spectral purity, that need to be deeply investigated when aiming to make these innovative laser sources suitable for high-resolution spectroscopy and metrology. This paper is a review of our efforts towards QCL-based high-resolution spectroscopy and of our experimental investigation of QCLs' frequency noise, aimed to discover the ultimate performances attainable by QCLs and to develop the exper- imental techniques required to achieve them. Our results, confirmed by several independent measurements, show that QCLs have a very small intrinsic linewidth buried under a large frequency-noise background. The development of appropriate frequency stabilization techniques will make QCLs well suited for high-resolution spectroscopy and metrology in the mid and far IR. C 2010 Society of Photo-Optical Instrumentation Engineers.

Digital Laser Frequency Stabilization Using an Optical Cavity

Pound-Drever-Hall locking is a high performance technique for laser frequency stabilization. Radio frequency optical modulation combined with electronic demodulation provides a feedback signal to lock a laser's frequency to a cavity resonance. Although the demodulation and feedback system are typically implemented using analog electronics, digital implementations are now possible thanks to recent advances in digital signal processing. Aside from flexibility, digital systems have improved performance at low frequencies where electronic noise may be a problem. In this paper we analyze several noise sources that appear in a digital Pound-Drever-Hall locking loop and estimate their effect on the performance of the stabilization system. Furthermore, we implement a digital Pound-Drever-Hall scheme and characterize the feedback performance by beating two lasers locked to a single cavity. A relative frequency noise floor of 0.2 Hz/¿(Hz) above 3 Hz was measured, giving an upper bound on the noise performance of the digital system.

Arm locking with Doppler estimation errors

At the University of Florida we developed the University of Florida LISA Interferometer Simulator (UFLIS) in order to study LISA interferometry with hardware in the loop at a system level. One of the proposed laser frequency stabilization techniques in LISA is arm locking. Arm locking uses an adequately filtered linear combination of the LISA arm signals as a frequency reference. We will report about experiments in which we demonstrated arm locking using UFLIS. During these experiments we also discovered a problem associated with the Doppler shift of the return beam. The initial arm locking publications assumed that this Doppler shift can perfectly be subtracted inside the phasemeter or adds an insignificant offset to the sensor signal. However, the remaining Doppler knowledge error will cause a constant change in the laser frequency if unaccounted for. Several ways to circumvent this problem have been identified. We performed detailed simulations and started preliminary experiments to verify the performance of the proposed new controller designs.

Frequency stabilized three mode HeNe laser using nonlinear optical phenomena

Accurate and traceable length metrology is employed by laser frequency stabilization. This paper describes a laser frequency stabilization technique as a secondary standard with a fractional frequency stability of 5.2x10(-10) with 2 mW of power, suitable for practical applications. The feedback stabilization is driven by an intrinsic mixed mode signal, caused by nonlinear optical phenomena with adjacent modes. The mixed mode signals are described theoretically and experimentally verified.

Remote Frequency Stabilization Technique by Using MZI-AWG and Supervisory Frame Transfer in Coexistence-Type WDM-PON

We propose a remote frequency stabilization technique for upstream signals in coexistence-type wavelength-division-multiplexed passive optical networks (WDM-PONs). It is based on dithering the transmission spectra of the demultiplexer to create a frequency monitor; supervisory frames that contain frequency information are transferred from the central office to the optical network unit through a downstream signal. To achieve a wide capture range for feedback control, the DEMUX is built as a new planer lightwave circuit that incorporates a Mach-Zehnder interferometer (MZI) and two arrayed waveguide gratings (AWGs). Experiments show that the deviation in laser-diode (LD) frequency can be suppressed to plusmn3.5 GHz by using the supervisory frames with the fixed time interval of 2 ms even though the LD frequency is expected to drift by plusmn25 GHz against the ITU-T grid over its lifetime. An analytical estimation also shows that the proposed technique can effectively reduce the power budget required for the transceivers for upstream signal transmission with no significant decrease in the main signal bandwidth.

Doppler-free polarization spectroscopy with a quantum cascade laser at 43 µm

We report on what we believe to be the first Doppler-free polarization spectroscopy with a quantum cascade laser. A hot-band CO(2) transition around 4.3 microm wavelength has been used to test the potential of the technique for high-resolution spectroscopy and wide-bandwidth quantum cascade laser frequency stabilization.

A frequency stabilization technique for diode lasers based on frequency-shifted beams from an acousto-optic modulator

We present a simple method for diode laser frequency stabilization that makes use of a Doppler-broadened vapor cell absorption signals of two frequency-shifted laser beams. Using second-order-diffracted, double-passed beams from an acousto-optic modulator, we achieve a frequency separation roughly equal to the Doppler half width. The differential transmission signals of the two beams provide an error signal with a very large linear feature, allowing frequency stabilization over a range of greater than 1 GHz by means of standard proportional-integral-derivative servo feedback to the piezoelectric control of the grating in our external cavity diode laser. We have applied this technique to two different diode laser systems, one used to lock to the 410 nm E1 transition in indium and another for locking to the M1/E2 transition in thallium at 1283 nm. In both cases the technique reduces frequency fluctuation to roughly 1 MHz over time scales from 10(-3) to 10(2) s.

Centralized Frequency Stabilization With Wide Capture Range Using MZI-AWG in DWDM-PON

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This letter proposes a centralized frequency stabilization technique for uplink signals in dense wavelength- division-<?Pub _nolinebreak=""?> multiplexed passive optical networks; it is based on dithering the transmission spectra of the demultiplexer. To ensure that the proposed technique achieves a wide capture range, a planar lightwave circuit is developed that incorporates a Mach–Zehnder interferometer and two arrayed waveguide gratings. Experimental results show that the wide capture range of <emphasis emphasistype="italic"><formula formulatype="inline"> <tex Notation="TeX">$\pm$</tex></formula></emphasis>25 GHz is established without any significant power penalty. </para>

Laser frequency stabilization for atom cooling and magnetic-field compression of the trap

We report a frequency stabilization technique of a diode laser in a Doppler-free atomic transition used for obtaining the magneto-optical trapping of Rb atoms. This technique, based on side locking to an atomic transition using a servo controller, is very simple and can be implemented straightforwardly to lock the laser at the red detuned frequency position required for laser cooling experiments. The number of trapped atoms and the temperature of the cold cloud have been determined. The effect of trapping the magnetic field on the cloud radius has also been analyzed.

Laser frequency stabilization using a balanced bi-polarimeter

We report on a Doppler-free polarization spectroscopy based technique of laser frequency stabilization using a balanced bi-polarimeter set-up. Two linearly polarized weak laser beams are used to probe birefringence induced by two oppositely circularly polarized strong pump beams in a vapour cell. Subtraction of balanced polarimeter signals obtained from the two probe beams results in a background-free dispersion-like reference signal without frequency modulation. The dispersion-like signal corresponding to the closed transition 5 2S1/2 (F=2) →5 2P3/2 (F′=3) of 87Rb was used for frequency locking of a diode laser. The frequency fluctuations and the drift were measured to be less than 0.25 MHz and 0.02 MHz, respectively, for an observation period of more than 10 hours.

Phase locking to a LISA arm: first results on a hardware model

We present the first experimental confirmation of the so-called ‘self-phase-locked delay interferometry’. This laser frequency stabilization technique consists basically in comparing the prompt laser signal with a delayed version of itself that has been reflected in another LISA satellite 5 × 109 m away. In our table-top experiment, the phase of a voltage-controlled oscillator is stabilized by means of a control loop based on this technique. In agreement with the theory, the measured unity gain frequency is not limited by the inverse of the used delay (1.6 µs). In the time domain, the system also behaves as predicted, including the appearance of a quasi-periodic ‘ringing’ just after the lock acquisition, which decays exponentially. Its initial amplitude is smaller when the loop gain is slowly ramped up instead of suddenly switched on.

Stabilization, injection and control of quantum cascade lasers, and their application to chemical sensing in the infrared

Quantum cascade lasers (QCLs) are a relatively new type of semiconductor laser operating in the mid- to long-wave infrared. These monopolar multilayered quantum well structures can be fabricated to operate anywhere between 3.5 and 20 μm, which includes the molecular fingerprint region of the infrared. This makes them an ideal choice for infrared chemical sensing, a topic of great interest at present. Frequency stabilization and injection locking increase the utility of QCLs. We present results of locking QCLs to optical cavities, achieving relative linewidths down to 5.6 Hz. We report injection locking of one distributed feedback grating QCL with light from a similar QCL, demonstrating capture ranges of up to ±500 MHz, and suppression of amplitude modulation by up to 49 dB. We also present various cavity-enhanced chemical sensors employing the frequency stabilization techniques developed, including the resonant sideband technique known as NICE-OHMS. Sensitivities of 9.7×10 −11 cm −1 Hz −1/2 have been achieved in pure nitrous oxide.

Ultimate linewidth reduction of a semiconductor laser frequency-stabilized to a Fabry-Pérot interferometer.

We report a theoretical dynamical analysis on effect of semiconductor laser phase noise on the achievable linewidth when locked to a Fabry-Pérot cavity fringe using a modulation-demodulation frequency stabilization technique such as the commonly used Pound-Drever-Hall frequency locking scheme. We show that, in the optical domain, the modulation-demodulation operation produces, in the presence of semiconductor laser phase noise, two kinds of excess noise, which could be much above the shot noise limit, namely, conversion noise (PM-to-AM) and intermodulation noise. We show that, in typical stabilization conditions, the ultimate semiconductor laser linewidth reduction can be severely limited by the intermodulation excess noise. The modulation-demodulation operation produces the undesirable nonlinear intermodulation effect through which the phase noise spectral components of the semiconductor laser, in the vicinity of even multiples of the modulation frequency, are downconverted into the bandpass of the frequency control loop. This adds a spurious signal, at the modulation frequency, to the error signal and limits the performance of the locked semiconductor laser. This effect, reported initially in the microwave domain using the quasistatic approximation, can be considerably reduced by a convenient choice of the modulation frequency.

Frequency stabilization of millimeter-wave subcarrier using laser heterodyne source and optical delay line

A frequency stabilization technique for laser heterodyne sources that is applicable to fiber-radio systems is experimentally demonstrated, It employs an optical delay line as a frequency reference instead of a millimeter-wave oscillator. A subcarrier signal with a 12.3-MHz deviation was generated in a 10-GHz band by using commercial-grade DFB-LDs.

Transmission-line stabilized monolithic oscillators

A novel technique for frequency stabilization and phase-noise reduction of monolithic oscillators is presented in this paper. It employs simple transmission-line resonators, which are many wavelengths long to increase the oscillator quality factor. Monolithic oscillators at 20 and 40 GHz are realized for the application of this technique. Phase noise reduction of more than 20 dB was achieved for both oscillators. The single-sideband phase noise obtained was -100 dBc/Hz at 100-kHz offset for the 20-GHz oscillator and -90 dBc/Hz at 1-MHz offset for the 40-GHz oscillator. The approach is implemented by using readily available transmission lines, which are open- or short-circuited at one end and connected to the monolithic-microwave integrated-circuit (MMIC) oscillator at the other end. Thus, it presents significant potential in the development of low-cost MMIC oscillators with enhanced noise performance.

Frequency Stabilization of Waveguide CO2 Laser by a Digital Technique

In this work we present a simple technique for laser frequency stabilization, based on Digital signal processing. The technique is used to stabilize a waveguide CO2 laser of wide tunability by using three kinds of reference signals: the CO2 laser ouptut power, an infrared absorption optoacoustic signal and the output power of a Far-Infrared optically pumped molecular laser.

1/ f phase noise in a transistor and its application to reduce the frequency fluctuation in an oscillator

The 1/ f noise generating mechanism is discussed with the diffusion coefficient fluctuation in the base. Results of noise measurements of the phase and amplitude or average current noises and their correlation coefficient in collector current are presented. A possible noise reduction method is suggested, using the correlation between the phase and amplitude fluctuations simultaneously. Also, measurement of the frequency and average current fluctuations along with their correlation coefficient for transistor oscillators is presented. In this case, it is found that both fluctuations correlate with each other at low frequency. A frequency stabilization technique is discussed and a new method to reduce the frequency fluctuation is proposed for an oscillator using the correlation.

Excitonic transitions in Si δ-doped GaAs studied with wavelength modulated lateral photoconductivity

The New Photothermal Wavelength Modulated Photocurrent (PTWMPC) technique was recently used and demonstrated as a simple method for studying the excitonic transitions under the bandgap in Si δ-doped GaAs. The spectroscopic aspects of PTWMPC have been theoretically and experimentally studied. The PTWMPC phenomena and the nature of registered features are explored in details. The PC spectrum line shape, the full width at half-maximum intensity dependence, the chopping frequency and the excitonic peaks position intensity dependence are discussed. The PTWMPC can be proposed as a simple external resonator optoelectronic technique for laser diode frequency stabilization and the Si δ-doped GaAs cell is useful as a device with sharp reference frequency absorption peaks for metrological purposes.

Asymmetry studies of iodine resonances and realization of unperturbed molecular transition in a laser standard at 633 nm

A detailed study of the asymmetry of saturated absorption resonances in a laser wavelength standard operating at 633 nm is presented. Analysis of the results for the third and fifth harmonic frequency stabilization techniques has been performed. A new approach in the studies has made it possible to handle the problem of correlated pressure, modulation width and power shifts in the laser standard. As a result, the laser frequency shift due to the resonance asymmetry has been separated from the iodine pressure (inelastic collision) shift. The frequency shift associated with the resonance asymmetry has been shown to increase nonlinearly (quadratically) with the iodine pressure. Collisions with small angle of scattering are found to be the main reason for this effect. The unperturbed molecular transition frequency for the i-component of iodine has been found to be 473 612 214 858.5 ± 12.7 kHz. The validity of the method is confirmed by frequency measurements performed with a special standard, featuring ten times smaller line asymmetry than that of a conventional laser standard.

Accurate measurement of ultralow loss in a high-finesse Fabry-Perot interferometer using the frequency response functions

We describe the accurate measurement of ultralow loss in a high-finesse Fabry-Perot interferometer using a diode-pumped Nd:YAG laser locked to the longitudinal mode with an active frequency-stabilization technique. By measuring the resonance full width and the free spectral range with the frequency response functions, and by measuring the transmission efficiency on resonance, the finesse and the loss at 1064 nm are accurately measured to be 78100 ± 1200 (reflectance of 99.99598 ± 0.00006%) and 1 5.9 ±2.0 × 10−6 (15.9 ± 2.0 ppm), respectively.