Frequency-diverse structured light for optical gradient sensing using heterodyne interferometry

An heterodyne interferoaeter is constructed with a sodulated laser diode (LD) in a Michelson interferoaeter. Phase drift of beat frequency with teiperature and its affection aechanis is discussed and a method elisinating the affection aentioned above is proposed. 1 . OPTICAL HETERODYNE INTERFEROMETER Optical frequency of laser diode can be linearly aodulated by an injection current. An optical heterodyne interferoseter is constructed with a modulated laser diode in a Michelson interferoseter. If a difference exists between two arms in the interferometer a optical beat frequency phenoaenon will be forRed. The beatfrequency signal can be detected with a photoelectric device. The output signal can be described by the following equation: 1g10[1+cos(2xfbt+t)J (1) and fb is given by fb 6f''8Lf. I C (2) Where f is the aaxiRua optical frequency variation value of wxiulated laser fb the beat frequency value fa the odulated signal frequency L the optical path difference between two aras in the interferoaeter C optical wave speed. 2. VARIATION OF BEAT FREQUENCY PHASE WITh TEMPERATURE It is well known that the optical frequency of a laser diode is affected by the tesperature (in the equation (1)) ay be changed with the variation of central wavelength of . odulated optical frequency. t aay be expressed in the for. : Li A242Ai t 2ir( ) (3) AiA2 The plus and ainus . arks in the equation above are corresponding to the upward

The basic operation, implementation and control of femtosecond laser optical frequency combs will be presented. The application of frequency combs in various scientific and technological fields will be described. 150-word Biography: Scott Diddams received the B.A. in Physics from Bethel College, St. Paul, MN in 1989 and the Ph.D. degree in Optical Science from the University of New Mexico, Albuquerque in 1996. From 1996 through 2000, he did postdoctroral work at JILA, University of Colorado. In 1998 Dr. Diddams was awarded a National Research Council fellowship to work with Dr. John Hall (also at JILA) on the development of optical combs for frequency metrology. The techniques developed at that time have had a significant impact on the fields of ultrafast optics and optical frequency metrology. Since 2000, Dr. Diddams has been a staff member and project leader at the National Institute of Standards and Technology (NIST) where he and his coworkers have continued the development of optical frequency combs for optical clocks, waveform generation, low-noise microwave synthesis, spectroscopy and other applications. Dr. Diddams was a co-recipient of a Department of Commerce gold medal for revolutionizing the way frequency is measured and was additionally the recipient of the Presidential Early Career Award in Science and Engineering (PECASE) for his work on optical frequency combs. He is a Fellow of the OSA and APS and a member of IEEE. Article not available.

The application of two gain switched optical frequency combs (OFCs) in dual comb gas phase spectroscopy is demonstrated. We report on the stability analysis of the wavelength and power of individual comb lines of the two OFCs. The examination reveals that a maximum wavelength fluctuation of <2.5 pm and a maximum peak power fluctuation of ∼0.3 dB is achievable for the OFCs. The radio frequency (RF) beat tone spectrum shows the standard deviation of the peak power of an individual beat tone from the mean is as low as ∼0.14 dB with negligible frequency fluctuations. In a proof-of-principle experiment the dual comb system is applied to the detection of hydrogen sulphide (H2S) with a detection sensitivity of (740 ± 160) ppmv, demonstrating its excellent frequency and power stability. The dual OFCs can in principle be monolithically integrated and thus enable the development of compact, cost-efficient dual comb devices, for the detection of multiple trace gas species or isotopologues.

As a bridge connecting microwave frequency and optical frequency, femtosecond laser has important significance in optical frequency measurement. Compared with the traditional Ti-sapphire femtosecond optical frequency comb, with the advantages of compact structure, strong anti-interference ability and low cost, the fiber femtosecond optical frequency comb has a wider application prospect. An experiment of spectrum broadening in a highly nonlinear photonic crystal fiber pumped by an Er-fiber mode-locked femtosecond laser is studied in this paper. Based on optical amplification and frequency doubling, the central wavelength of the output spectrum is 780nm and the average power is 232mW. With the femtosecond pulses coupled into two different photonic crystal fibers, the coverage of visible spectrum is up to 500nm-960nm. The spectral shape and width can be optimized by changing the polarization state for satisfying the requirments of different optical frequencies measurement.

The precise measurement of difference frequencies between two lasers in a terahertz (THz) region is a basic technique for optical frequency standards and for many other applications, such as dense WDM communication systems. Frequency difference measurements based on the beat signal between two lasers become quite difficult when the frequency difference exceeds the detector bandwidth. An optical frequency comb (OFC) generator with an electro-optic (EO) phase modulator inside a Fabry-Perot cavity can overcome this problem. The OFC generator creates a large number of sidebands, which can be applied for frequency markers. In conventional OFC generation, modulation sidebands appears on both sides of the carrier, which is a disadvantage for the efficient use of frequency bandwidth. The limit of the frequency difference measurement with this scheme depends on the power degradation in the sidebands. We propose an alternative technique for generating OFC which employs a dual-electrode Mach-Zehnder Modulator (D-MZM) inside a fiber recirculating loop to generate optical single sideband (SSB). This enables the efficient use of the frequency bandwidth. An optical amplifier is used in the loop to compensate for the losses.

We describe a quantitative method for measuring the phase of a propagating wave field in three dimensions by use of a scanning optical-fiber interferometer. Because phase modulation in the reference arm is exploited, this technique is insensitive to large variations in the intensity of the field being studied and is therefore highly suitable for measurement of phase within spatially confined optical beams. It uses only a single detector and is not reliant on lock-in electronics. The technique is applied to the measurement of the near field of a cleaved optical fiber and is shown to produce results in good agreement with theory.

We study optical frequency comb (OFC) generation using cascaded injection of semiconductor lasers in this work. The OFC generation system is operated in two cascaded optical injection stages. When a master laser optically injects into the first stage with proper injection power and frequency, period-one (P1) dynamics are invoked in an optically injected semiconductor laser of the first stage. Another semiconductor laser in the second stage is then optically injected by the P1 dynamics. With proper injection power adjusted in the second stage, the P1 dynamics are regenerated, and the semiconductor laser relaxation oscillations (ROs) become undamped so that subharmonic oscillations appear. Because a subharmonic oscillation frequency is half of an oscillation frequency of the P1 dynamics, extra optical frequency components appear in the middle of the adjacent optical frequency components of the P1 dynamics, thus signaling OFC generation. The OFC signals exhibit at least 15 comb lines, resulting in a bandwidth greater than 140 GHz. Microwave comb signals are obtained after photodetection, although the microwave linewidth is on the order of a few megahertz because of the semiconductor laser noise. Thus, we propose a cascaded injection-locking scheme to stabilize the P1 dynamics and OFC signals. We have demonstrated pure microwave generations with a linewidth of less than 3 Hz and low phase noise.

Optical frequency combs [1-3] provide equidistant frequency markers in the infrared, visible and extreme ultra-violet [4,5]. Early work by T.W. Hansch recognized that the periodic pulse train of a mode-locked laser [6,7], which intrinsically provides in Fourier domain an output spectrum that constitutes an optical frequency comb, can be used to measure unknown optical frequencies. An optical frequency comb is generally characterized by its two degrees of freedom, the mode spacing frep, which in a pulsed laser is given by the pulse repetition rate [8], as well as the carrier envelope offset frequency fceo, which determines the frequency offset of the comb teeth from integer multiples of the repetition rates.In time domain, fceo describes the phase slippage of the pulse envelope with respect to the carrier. Thus, any comb line of the resulting output spectrum can be expressed by fm = fceo + mfrep, where m is an integer. Measurement and control of frep and fceo allows to phase-coherently link optical frequencies across the entire spectrum spanned by the comb [8,9]. In this case, every comb tooth is uniquely determined by the experimentally controlled quantities fceo and frep. Therefore, any arbitrary optical frequency within the spectrum of the comb may be synthesized, or, vice versa, any given optical frequency within this spectrum may be phase-coherently compared to the frequencies fceo and frep in the RF domain, which can be referenced to microwave time standard [10,11]. This unique ability has made the frequency comb a revolutionary tool in metrology and laser spectroscopy, and, serving as a clockwork mechanism, has enabled the development and practical use of time standards in the optical domain [2,3,12,13]. Beyond these advances in metrology and precision measurements, frequency combs have given new impetus to applications such as broadband laser-based gas sensing [14-16] or cavity ring down spectroscopy [17].

Practical optical frequency measurement system around 1.5 μm based on an acetylene-stabilized laser-locked optical frequency comb

We demonstrate dual-comb spectroscopy with relatively phase-locked two frequency combs, instead of frequency combs firmly fixed to the absolute frequency references. By stabilizing two beat frequencies between two mode-locked lasers at different wavelengths observed via free-running continuous-wave (CW) lasers, two combs are tightly phase locked to each other. The frequency noise of the CW lasers barely affects the performance of dual-comb spectroscopy because of the extremely fast common-mode noise rejection. Transform-limited comb-resolved dual-comb spectroscopy with a 6 Hz radio frequency linewidth is demonstrated by the use of Yb-fiber oscillators.

When optical frequency fields are used to couple a ground state to a Rydberg state, the resonant dipole coupling is to a low angular momentum state. Higher angular momentum states are typically thought not to play a role in the excitation. The extremely large dipole matrix elements coupling Rydberg states of the same $n$ but differing $l$, however, allow optical fields of modest strengths to produce Rabi frequencies larger than optical frequencies. We demonstrate that these optical fields can therefore readily excite the higher angular momentum states, and we examine the consequences of this coupling.

Examination of various issues related to the generation and application of optical frequency combs (OFCs) is an important branch of modern optoelectronics. Some of the proposed OFC generation methods apply acousto-optic (AO) devices. The AO devices are used either as the element devoted to the OFC phase stabilization or they play the role of an optical radiation frequency shifting element in the frequency-shifting loop (FSL) scheme. The results of two experiments related to the application of AO cells in the FSL scheme are represented in this paper. The first experiment confirms the previously proposed effect of AO mismatch influence on all the OFC characteristics. The second experiment shows the possibility of tunable AO dual-comb downconversion with a single AO device.

The analogy between optical frequencies and RFs leads to a novel technique for RF photonic signal processing using a femtosecond laser comb modulated by a traveling-wave tunable filter, such as an acoustooptic tunable filter (AOTF) or a novel electrooptic tunable filter (EOTF). In this new scheme, the recent history of an applied RF waveform to-be-processed slides into the tunable filter and a femtosecond pulse train diffracts off the moving acoustooptically or electrooptically induced dielectric grating, producing a shaped optical pulse train with each shaped pulse as a compressed replica of the RF waveform contained within the device aperture that is sped up by the ratio between the center optical and RF frequency. For a CW RF tone input, only a narrowband group of the frequency comb lines in the incoming laser comb is spectrally filtered and modulated due to the phase-matching condition in the filter and is simultaneously Doppler shifted by that RF due to the traveling-wave grating obeying the conservation of energy. This allows us to use the traveling-wave tunable filter as a spectrally mapped Doppler-shifted modulator that encodes different RF components onto the corresponding optical frequency comb lines. RF signal processing can then be performed by using optical techniques to manipulate the spectrally modulated laser comb. To reconstruct the processed RF signal, the Doppler-shifted and optically processed pulse train is heterodyne detected by beating with a reference femtosecond pulse train from the same laser source. A high repetition rate femtosecond laser comb is modulated by an AOTF to experimentally demonstrate this novel RF photonic signal processing technique. We demonstrate an RF tunable bandpass/notch filter, an RF down-converter, and an RF jammer nuller as novel applications of ultrafast lasers.

A recirculating loop that includes an acousto-optic frequency shifter (AOFS) constitutes a versatile procedure to generate an optical frequency comb (OFC) from a continuous wave (cw) laser. This scheme, implementable with conventional fiber equipment, is capable of producing spectra composed of at least hundreds of coherent lines using a single-stage system. In this letter, we summarize the method to generate these acousto-optic frequency combs and briefly describe a simple model to optimize their performance. We also overview their main reported applications, paying special attention to those based on the peculiarities exhibited by the optical field at the output of the fiber loop.

Length is one of the fundamental physical quantities and its precise measurement is very important for science and technology. Laser-based distance metrology technique is a powerful tool, which is widely applied in geodetic monitoring, environmental monitoring and precision measurement engineering including in space applications. Since the invention of the optical frequency comb (OFC) at the beginning of this century, it has been used as a versatile tool for many applications, such as frequency metrology, spectroscopy, etc. OFC has played an important role in distance metrology enabling the developed techniques to achieve high accuracy in long distance measurement. In the time domain, OFC is a pulse train, which can measure distance with time of flight method or correlation detection method. In the frequency domain, spectrum of OFC consists of a series of discrete lines with equal frequency difference. Methods of dispersive interferometry, multi-heterodyne interferometry, and multi-wavelength interferometry are the techniques proposed and demonstrated in spectral domain distance metrology. Though distance metrology techniques based on OFC have been developed for tens of years, there are still challenges that need to be overcome. In most applications the measurements are completed in the air, where the results are influenced by refractive index of air. As the OFC is a multi-wavelength laser source, refractive index of air is difficult to determine. The other challenge is complexity and high cost of measurement system. Until now, to my knowledge, there is no commercial rangefinder which is based on OFC. The large size, complex configuration and high price are the bottlenecks restricting OFC’s entrance into industrial applications.