Intermode Beat Research Articles

Refractivity corrected distance measurement using the intermode beats derived from a supercontinuum.

Simultaneous distance measurements on two or more optical wavelengths enable dispersion-based correction of deviations that result from insufficient knowledge of the refractive index along the signal propagation path. We demonstrate a supercontinuum-based approach for highly accurate distance measurements suitable for such an inline refractivity compensation. The distance is estimated from the phase delay observations on the intermode beats. We use a supercontinuum (SC) coherently broadened from a 780 nm frequency comb and spanning the spectral range of 570-970 nm. Experiments are performed on the 590 and 890 nm wavelength bands filtered from the SC spectrum. Results show distance measurements with standard deviations of around 0.01 mm at 50 m, and a distance-dependent component below 0.2 ppm on the individual spectral bands. Distance residuals compared to a reference interferometer are on the order of 0.1 ppm for displacements up to 50 m. Controlled pressure-induced refractivity variations are created over a length of 15 m, resulting in an optical path length change of 0.4 mm. Using the two-color method, we demonstrate refractivity-corrected distance measurement with a standard deviation of around 0.08 mm for a 60 s averaging window. The current experimental configuration can be easily extended to distance measurements on more than two wavelengths. The results highlight its potential for practical long-distance measurements through inline refractivity compensation.

Supercontinuum-based hyperspectral LiDAR for precision laser scanning.

Hyperspectral LiDAR enables non-contact mapping of the 3D surface geometry of an object along with its spectral reflectance signature and has proved to be effective for automated point cloud segmentation in various remote sensing applications. The established hyperspectral LiDAR methods offer a range precision of a few mm to a few cm for distances exceeding several meters. We propose a novel approach to hyperspectral LiDAR scanning based on a supercontinuum (SC) coherently broadened from a 780 nm frequency comb. It provides high precision distance measurements along with target reflectance over the 570-970 nm range of the SC output. The distance measurements are carried out by monitoring the differential phase delay of intermode beat notes generated by direct photodetection, while the backscattered light spectrum is acquired using a commercial CCD spectrometer with 0.16 nm resolution across the 400 nm bandwidth of the SC output. We demonstrate a measurement precision below 0.1 mm for a stand-off range up to 50 m on a diffuse target with around 89% reflectance. The measured relative accuracy as compared to a reference interferometer is on the order of 10-5 for distances up to 50 m. Initial results also indicate spectrum-based material classification within a 3D point cloud using a linear support vector machine. The results highlight the potential of this approach for joint high-precision laser scanning and automated material classification.

Near-Full Current Dynamic Range THz Quantum Cascade Laser Frequency Comb.

The present study proposes a terahertz quantum cascade laser frequency comb (THz QCL FC) with a semi-insulated surface plasma waveguide characterized by a low threshold current density, high power and a wide current dynamic range. The gain dispersion value and the nonlinear susceptibility were optimized based on the combination of a hybrid bound-to-continuum active region with a semi-insulated surface plasmon waveguide. Without any extra dispersion compensator, stable frequency comb operation within a current dynamic range of more than 97% of the total was revealed by the intermode beat note map. Additionally, a total comb spectral emission of about 300 GHz centered around 4.6 THz was achieved for a 3 mm long and 150 µm wide device. At 10 K, a maximum output power of 22 mW was obtained with an ultra-low threshold current density of 64.4 A·cm-2.

Methods of Mode Generation Inside Hollow Core Photonic Crystal Fibers

Microspheres trapped inside Hollow Core Photonic Crystal Fibers (HCPCF) could provide a way to monitor the temperature in hydrogen combustors, thereby helping to provide a warning system for flashback and thermoacoustic oscillations that can lead to expensive combustor damage. The temperature of a particle trapped in a HCPCF may be extracted by the analysis of the particles' motion, which can be in turn controlled by opportune manipulation of the spatial intensity profile of the light in HCPCF. To this aim, an intermodal beating intensity pattern may be created inside the HCPCF using a mixture of LP01 and LP11 modes. In this work, methods of generating the optical modes, which involve the use of spatial light shaping techniques, are presented and analysed. This is an important step to producing a controllable intermodal beating pattern.

Real-time suppression of random phase drift for laser ranging with high-frequency intermode beats

Laser ranging with frequency comb intermode beats (IMBs) has been suffering from random phase drifts (RPDs) for two decades. In this study, we reveal the influence of signal transmission path on the RPDs and propose a real-time suppression method using two IMBs of similar frequencies from different combs. As the two IMBs obtain similar RPDs during their transmission through same signal paths, the RPD of the original probing signal IMB is suppressed by deducting the RPD of the newly added local IMB in real time. In our experiments, a real-time suppression of RPDs is achieved using IMBs of 1001 and 1000 MHz. For the sampling time of 100 s, the effect of 19-fold suppression has been achieved. The proposed method provides a new solution for the long-standing phase drift problem in laser ranging with comb IMBs.

Analog FM free-space optical communication based on a mid-infrared quantum cascade laser frequency comb.

Quantum cascade laser frequency combs are nowadays well-appreciated sources for infrared spectroscopy. Here their applicability for free-space optical communication is demonstrated. The spontaneously-generated intermodal beat note of the frequency comb is used as carrier for transferring the analog signal via frequency modulation. Exploiting the atmospheric transparency window at 4 µm, an optical communication with a signal-to-noise ratio up to 65 dB is realized, with a modulation bandwidth of 300 kHz. The system tolerates a maximum optical attenuation exceeding 35 dB. The possibility of parallel transmission of an independent digital signal via amplitude modulation at 5 Mbit/s is also demonstrated.

Frequency axis for swept dual-comb spectroscopy with quantum cascade lasers.

In dual-comb spectroscopy, there is a one-to-one map between the frequencies of the measured beat notes and the frequencies of the optical comb lines. Its determination usually involves the use of one or more reference lasers with known frequencies. Quantum cascade laser frequency combs, however, are often operated in a free-running mode, and without a reference, the determination of the RF-to-optical frequency map is not trivial. Here, we propose a method by which the comb shift is measured with an unbalanced Mach-Zehnder interferometer, and the spectral point spacing is determined through the intermode beat measured on the laser electrodes. The frequency axis is accurate within ∼ 0.001 cm-1.

Intracavity optical parametric oscillator: Model of dynamic system with different values of time delay for pump and signal radiation

Most of intracavity pumped optical parametric oscillators (OPO) are made nowaday according to a scheme with a single-resonance OPO located in the cavity of a pump laser. Usually the cavities of the pump and OPO (signal) emission have different values of round-trip time (delay). Aim of the study is therefore to build up the mathematical model of intracavity optical parametric oscillator (ICOPO) considered as a time-delay dynamic system with two values of delay in both cavities (the pump and signal). Methods. The model allows to analyze the steady state (equilibrium point) of the dynamic system and its stability with the help of characteristic equation’s solution. Results. Countless set of the characteristic equation roots is shown to consist of complex-conjugate pairs with imaginary parts which are nearly multiples of intermode beat frequencies in the pump and signal cavities. The diagram of stability depending on the position of nonlinear crystal in the resonator was built on the parameter plane. Features of the plane partition into the areas of stability/instabilty vs behavior of the characteristic equation roots are examined. Discussion. The results of the study allow to consider an ICOPO as the time-delay dynamic system thus adding to the physical picture of intracavity parametric oscillators.

High-resolution spectral analysis of long single-frequency pulses generated by a self-sweeping Yb-doped fiber laser

Continuous wave self-sweeping operation is demonstrated in a classical ring cavity configuration of Yb-doped fiber laser with a saturable absorber, which is usually used to obtain single-frequency lasing. Using heterodyne-based high-resolution spectral analysis we demonstrate that the laser generates a series of long (up to 1 ms) rectangular pulses consisting of single frequency radiation (the linewidth <0.7 MHz). At the same time, the lasing frequency changes between pulses regularly in a stepwise manner by one intermode beating frequency within a self-sweeping range of 100 pm. The effect of cavity configuration on pulse parameters (duration and chirp) is investigated. We also demonstrate that saturable absorbing mirrors design can be used to control the self-sweeping direction and value of the frequency jump.

Coherently-averaged dual comb spectrometer at 7.7 µm with master and follower quantum cascade lasers.

We demonstrate coherent averaging of the multi-heterodyne beat signal between two quantum cascade laser frequency combs in a master-follower configuration. The two combs are mutually locked by acting on the drive current to control their relative offset frequency and by radio-frequency extraction and injection locking of their intermode beat signal to stabilize their mode spacing difference. By implementing an analog common-noise subtraction scheme, a reduction of the linewidth of all heterodyne beat notes by five orders of magnitude is achieved compared to the free-running lasers. We compare stabilization and post-processing corrections in terms of amplitude noise. While they give similar performances in terms of signal-to-noise ratio, real-time processing of the stabilized signal is less demanding in terms of computational power. Lastly, a proof-of-principle spectroscopic measurement was performed, showing the possibility to reduce the amount of data to be processed by three orders of magnitude, compared to the free-running system.

Intensity Noise of Semiconductor Lasers Induced by Inter-Mode Beating

Beating between the lasing main mode and the non-lasing side modes is one of the intensity noise sources of single longitudinal mode lasers. Its characteristics were investigated experimentally recent years. This paper presents a theoretical study of mode-beating noises in semiconductor lasers, and intends to explain the experimentally observed phenomena. The effects of coherent population oscillation (CPO) induced by mode beating on gain coefficient and on reflective index are analyzed based on Maxwell-Bloch equation and rate equations. The power ratio of the side mode over its conjugation mode generated by four wave mixing (FWM) is discussed; formulas of amplitude and frequency of mode beating intensity noise are given with relations to pump levels and mode orders. The obtained results are suitable for the edge emitting diode lasers and the external cavity lasers. The line shape of mode beating intensity noises is discussed based on CPO-induced phase noise, giving explanations of the experimentally observed phenomenon of multiple sub-peak spectra. The roles of intensity noise reduction by nonlinear optical absorber (NLA) inserted in the laser cavity are described. More factors related to the mode beating noise are discussed qualitatively, such as spectral hole burning of gain (SHB), the effect of external cavity, and higher order FWM.

Mode-locked short pulses from an 8\xa0\u03bcm wavelength semiconductor laser

Quantum cascade lasers (QCL) have revolutionized the generation of mid-infrared light. Yet, the ultrafast carrier transport in mid-infrared QCLs has so far constituted a seemingly insurmountable obstacle for the formation of ultrashort light pulses. Here, we demonstrate that careful quantum design of the gain medium and control over the intermode beat synchronization enable transform-limited picosecond pulses from QCL frequency combs. Both an interferometric radio-frequency technique and second-order autocorrelation shed light on the pulse dynamics and confirm that mode-locked operation is achieved from threshold to rollover current. Furthermore, we show that both anti-phase and in-phase synchronized states exist in QCLs. Being electrically pumped and compact, mode-locked QCLs pave the way towards monolithically integrated non-linear photonics in the molecular fingerprint region beyond 6 μm wavelength.

High-precision long-distance measurement with an intensity-modulated frequency comb.

We describe an improved synthetic wavelength method for high-precision long-distance measurement with a repetition-rate-locked femtosecond laser modulated by a fiber Mach-Zehnder electro-optic intensity modulator. Harmonics of the repetition rate accompanied with modulating sidebands will be generated via intermode beating, which will be utilized for high-precision ranging. The nonambiguity range is significantly extended with a relatively low modulation frequency, and the ambiguous distance is unwrapped by synchronous phase-shift measurements of a synthetic wavelength chain without any auxiliary measurement operation. Our experiment shows a precision better than 20 µm at 46 m range, and a high-precision translation stage is applied for preliminary test and proof-of-principle demonstration. The demonstrated system is simple and can be easily integrated, and it will find widespread applications in large-scale metrology such as large-volume manufacturing and precision formation flying.

Actively Mode-Locked Semiconductor Laser with Feedback at an Intermode Frequency

This is a study of an active mode synchronization regime (mode locking) in a three-mirror semiconductor laser obtained by modulation of the laser current at the frequency of intermode beating of the external laser cavity through a feedback circuit. Under certain conditions stable active mode-locking is observed, and when the length of the external cavity is varied, corresponding tuning of the intermode beat frequency occurs. The main conditions under which this regime is attained are determined. The width of the intermode beating spectrum is found experimentally to be ~300 Hz for an intermode frequency of 300 MHz. The half width of the optical spectrum is ~4 nm.

Z shaped like resonator with crystal in the presence of flat mirror based standing wave ratio for optical antenna systems

&lt;span&gt;This study has outlined the Z resonator shaped with Brewster crystal in the presence of flat mirror for measuring the standing wave ratio. Stability parameter and beam radius are simulated versus thickness, refractive index of the crystal and first and second folding ranges. Beam radius variations are studied against phase angle and curvature radius of spherical mirror in T and S planes. Intermode beat frequency of the system is 216.276 MHz and total cavity length is 693.078 mm. It is important the standing wave ratio is dependent on stability parameter and beam radius variations. &lt;/span&gt;

Passively Mode-Locked 2.7 and 3.2 μm GaSb-Based Cascade Diode Lasers

The passively mode-locked type-I quantum well cascade diode lasers operating near 2.7 and 3.2 μm generated trains of the ∼10 ps long pulses with average power up to 10 mW. The devices based on laser heterostructures with reinforced carrier confinement requires increased reverse bias voltages applied to absorber sections to operate in mode-locked regime. The autocorrelation measurements showed that lasers generated strongly chirped pulses with temporal width an order of magnitude above transform limit. The application of the external feedback led to narrowing of the laser emission spectra accompanied by an order of magnitude reduction of the intermodal beat note linewidth. The multiheterodyne beat notes have been observed for devices stabilized by external feedback.

Long distance measurement by dynamic optical frequency comb.

In this paper, we propose a method aiming to measure the absolute distance via the slope of the inter-mode beat phase by sweeping the repetition frequency of the frequency comb. The presented approach breaks the inertial thinking of the extremely stable comb spacing, and the bulky phase-locking circuit of the repetition frequency is not required. In particular, the non-ambiguity range can be expanded to be infinite. To verify the performance of presented method, a series of distance experiments have been devised in different scenarios. Compared with the reference values, the experimental results show the differences within 25 µm at 65 m range in the laboratory, and within 100 µm at 219 m range out of the lab.

Picosecond pulses from a monolithic GaSb-based passive mode-locked laser

We present passive mode locking of a GaSb-based monolithic diode laser emitting at 2.2 μm with a fundamental repetition rate around 9.57 GHz. A pulse width of ∼2.4 ps is reconstructed by shifted wave intermode beat Fourier transform spectroscopy-measurements, yielding a time-bandwidth product of 1.8. Mode-locking is observed for a range of reverse bias voltages from 1.3 to 3.3 V and driving currents from 110 to 300 mA. The continuous wave output power is ∼17.5 mW with the absorber segment left floating and ∼4.5 mW at a reverse bias of 3.1 V in the mode-locked regime. The full-width-half-maximum of the radio frequency signal is measured for all operation conditions, with a minimum of 8.4 kHz.

Indications on self mode-locking in a broad area single-section quantum dot laser

Broad-area edge-emitting monolithic mode-locked semiconductor quantum dot lasers emitting at 1.26 μm could potentially serve as ideal sources for the generation of high power broad optical frequency combs for short-reach inter and intra data-center links. In this contribution, the inter-mode beat frequency of a 2 mm long InAs/InGaAs quantum dot laser with a broad-ridge waveguide are studied experimentally. Laser output power, radio-frequency and spectral domain analysis is performed. -3 dB spectral widths ranging from 2nm to 5.3 nm and the existence of an inter-mode beat frequency at 20.4 GHz with a signal-to-noise ratio from 2 dB up to 11 dB are experimentally confirmed for injection currents from 0.225 A to 1 A. Our results indicate a potential way towards high output power optical frequency comb generation by electrically injected monolithic semiconductor lasers.

Dynamic Intermode Beat Frequency Control of an Optical Frequency Comb Single Section Quantum Dot Laser by Dual-Cavity Optical Self-Injection

Dynamic frequency tuning of the 40.67 GHz intermode beat frequency of a 1255 nm emitting 1 mm long monolithic self mode-locked single section optical frequency comb InAs/InGaAs quantum dot laser across 70 MHz is experimentally demonstrated by fine-delay dual-cavity controlled all optical self-injection. Fiber-based macroscopic optical delay lengths are 9.4 m (round-trip time of 62.7 ns) and 16.5 m (round-trip time of 110.1 ns), the maximum studied microscopic delay tuning times are 40 ps and the optical self-injection strengths are below 0.02%. For selected delay times, the lowest intermode beat frequency line width amounts to 2 kHz indicating an improvement of carrier phase coherence by a factor of 700 as compared to the free-running laser. We validate these experimental results by a simple and universal stochastic time-domain model which is applied for the first time to model a self mode-locked quantum dot laser subject to optical self-injection. Modeling results are in good quantitative agreement.