Gabel and H ercher e x c e p t t h a t t h e pump l a s e r was fo cu sed d i r e c t l y onto a j e t stre a m o f Rhodamine 6 G and t h a t F a b r y - P e r o t é ta l o n s were in tr o d u c e d i n t o th e c a v i t y f o r f re q u e n c y n a rro w in g . The c r y s t a l used was ADA which can be 90° phase-m atch ed a t t h e l a s i n g w av elen g th s o f Rhodamine 6G, The c r y s t a l was 30 mm i n le n g t h and a n t i r e f l e c t i o n c o a te d fu se d s i l i c a f l a t s were a t t a c h e d to th e end f a c e s u s in g s i l i c o n e o i l (Bayer AG 10,000) U sing th e p rism a s a . t u n i n g e lem en t and in c l u d in g a 0 .1 mm t h ic k n e s s é t a l o n , th e l a s e r l i n e w i d th was 15 GHz. The maximum a v a i l a b l e pump power was 7 W from an arg o n io n l a s e r o p e r a t in g a t 514.5 nm. I t was found t h a t a t pump powers beyond ab o u t 4W th e dye l a s e r power s a t u r a t e d . T h is was a t t r i b u t e d to th erm al l e n s i n g i n th e dye j e t c a u sin g th e l a s e r to o p e r a t e i n h ig h o r d e r modes. By changing from an e th y le n e g ly c o l
In electro-optic diffraction devices transducers are not required and the device impedance is determined by the choice of material and device dimensions. Thus, given the correct choice of electro-optic medium (i.e. one without an absorption band in the spectral region of interest), the value of the time bandwidth product is dictated by the device dimensions only. The interaction length, L, affects the bandwidth by way of the condition that the grating wave should not change phase significantly (i.e. by less than 7i/4) during the time it takes an optical wave front to interact with the grating. For the travelling wave electro-optic diffractor the interaction length also dictates the diffraction efficiency per watt of rf power in the travelling wave. Thus there is a trade off between the diffraction efficiency of the device and its upper operating frequency (i.e. bandwidth).
polariton mode under small-angle noncollinear phase- matching conditions. In our first stage, nano-second pulse lasers were used to provide pump beams for DFG. Although wide tunable frequency range (0.3 - 7.5 THz) and high power (1.5 W, the maximum peak power) could be achieved, the frequency resolution was restricted to 500 MHz by Fourier transform limit. We also succeeded in developing a GaP ContinuousWave (CW)-THz SG in the same DFG method [13-16]. CW THz waves are su- perior to pulsed THz waves especially in terms of fre- quency resolution. However, frequency accuracy of the CW THz wave was not high because those of the pump laser beams were in free run mode. And still pump beam powers amplified by optical fiber amplifiers did not have enough stability against fine frequency sweep mainly because of interference at the power detector . We have improved the resolution of a GaP CW-THz SG by stabilization of the pump laser frequency and power. We have applied the improved SG to high resolution spec- trometer as a light source and evaluated the resolution and reproducibility by measuring water vapor absorption lines in vacuumed chamber.
Stimulated Raman Scattering (SRS) can be used to extend the wavelength coverage of common crystalline laser gain media [1,2]. This extended coverage benefits many applications especially when SRS is used in conjunction with Second Harmonic Generation (SHG) or Sum Frequency Generation (SFG) impacting medicine (retinal laser photocoagulation) , laser projection displays, and remote sensing (bathymetry or underwater detection) . All-solid-state continuous-wave (CW) Raman lasers are widely recognized as a practical and efficient way to produce laser outputs in the near infrared, visible and ultraviolet wavebands [5–11]. Typically, intracavity Raman lasers feature a separate Raman active crystal inserted inside a laser resonator. However, simpler cavity design and reduced optical losses can be achieved using a self-Raman configuration whereby the laser gain medium is also used as Raman gain medium [12,13]. However, this configuration amplifies the thermal lensing inherent to SRS and laser conversion and also facilitates the onset of excited-state absorption or impurity absorption [12,14]. Over the last decade, Neodymium-doped Ortho-Vanadates, such as Nd:YVO 4 and Nd:GdVO 4 have been
In recent years, we have investigated several all-solid-state alternatives to dyelasers [14– 17]. Among these different attempts, the intracavity frequency-doubled singly resonant optical parametric oscillator (SHG-SROPO) has given the most promising results . However, when dealing with the frequency noise of optical parametric oscillators, one must be aware of a fundamental difference between these sources and usual lasers based on stimulated emission: the parametric gain process is a coherent process. Consequently, due to energy conservation at the microscopic level, the frequency fluctuations of the pump laser are transferred to the signal and idler beams, and must be shared between these two beams. In the case of the SHG- SROPO, since the pump laser is a commercial frequency-doubled Nd:YVO 4 laser exhibiting
Significant scientific effort over recent years has aimed at the realization of terahertz (THz) frequency imaging sys- tems based on quantum cascade lasers (QCLs). Even though the continuous-wave output power of these devices can be as high as tens of mW, 1–3 there are still several technological issues that need to be addressed with the detection in order to realize a system that is sufficiently sensitive, as well as fast and compact. For this reason several groups have tested different detection techniques and configurations. Incoherent imaging systems have been demonstrated using Golay cells, pyroelectric detectors, cryogenically cooled bolometers, and commercial focal plane array microbolometric cameras. 4–7 More recently, a QCL-based imaging system was also dem- onstrated using an amorphous-silicon microbolometric cam- era that was specifically developed for operation in the THz range. 8 Coherent imaging techniques have also been reported in the literature, including a pseudo-heterodyne technique based on the mixing between the longitudinal modes of a multimode QCL, 9 self-mixing, 10 and exploiting the hetero- dyne mixing between a QCL and gas laser. 11 In the latter, the THz QCL was frequency-locked to the gas laser line in order to reduce the phase instability of the emitted radiation field and allow the application of an inverse synthetic aper- ture radar technique. Our work here is based on a coherent imaging technique that was first demonstrated by Loffler et al., 12 who used a harmonic of the repetition rate of a mode-locked Ti:Sa laser as a local oscillator and mixed it with a quartz-stabilized Gunn oscillator emitting at 0.6 THz. In that case, both sources were free running since their intrin- sic phase/frequency stability was sufficiently high. However, recently, it has been demonstrated that although THz QCLs have sub kHz quantum noise limited linewidths, 13,14 up to a
Stimulated Raman Scattering (SRS) is widely recognized as a practical and efficient approach to extend the spectral coverage of solid-state lasers operating in near infrared and visible spectrum especially when SRS is combined with second harmonic generation or sum frequency generation [1- 3]. However, the non-elastic nature of SRS results in the dissipation as heat in the Raman material of a significant portion of energy. This inevitably leads to undesired thermo-optical distortions and impacts the performance of the Raman laser (especially when the Raman crystal is inserted within the laser cavity). This additional thermal lensing scales directly with the Raman laser output power and has been identified as the main limitation in power scaling crystalline Raman lasers operating in the continuous-wave (CW) regime [2,4]. The use of low- loss, low-birefringence synthetic diamond can significantly reduce the effects of SRS-induced thermal lensing in CW Raman lasers [5-7]. In this paper, we propose a method to reduce both the effects of SRS and laser-induced thermal lensing leading to power scaling of crystalline Raman lasers. This method is based on a feedback control loop using adaptive optics (AO) which has been used to optimize the performance of solid-state lasers by compensating for the thermal lens effect within the laser gain medium [8-10].
olid-state lasers based on doped dielectric crystals are capable of unrivalled performance: from ultrashort pulses to kilowatt output powers; from multi-joule pulses to hertz- level linewidths. The output wavelength of a typical solid- state laser, however, is dictated by the electronic transitions of the dopant ion with very limited potential to engineer this to meet the requirements of a particular application. For this reason there is continuing interest in non-linear frequency conversion. The most flexible technique is the optical parametric oscillator . Tuning over hundreds of nanometres based on a fixed wavelength pump laser is possible. However, the requirement to phasematch the pump and generated waves means that optical parametric oscillators can be relatively complicated, particular for continuous-wave operation.
Various cavity designs have been considered, with high-quality optical materials and stabilised pump laser, for low threshold and widely tunable CW OPO operation. They range from standing wave to ring resonator, from monolithic, semi-monolithic cavity to conventional single-cavity, and dual cavity. These cavity designs have been employed for the CW operation of SRO, DRO, TRO, and even quadruply resonant OPO. However, most of the designs are for CW DROs. The monolithic cavity has excellent mechanical stability. Because the oscillation mode path is enclosed inside a single crystal and the resonant feedback is provided by total-internal-refiection or direct end-surface coating, absoiption and scattering losses, both due to multiple surface, are greatly reduced. However, this cavity design requires accurate polishing of the end face and lacks flexibility in changing of mirror coupling coefficients. Moreover, scanning the cavity length can be a problem, and high frequency stability pump laser is usually required. The dual cavity has the advantage of independent control of the two resonant fields to achieved continuous smooth tuning, but the intracavity beam splitter also increases the cavity loss and complexity of alignment. The widely used single-cavity DRO, however, provides the flexibility of changing the cavity free-spectral-range and output coupling coefficient. Although it has less mechanical stability compared with monolithic devices, single-cavity design could be well matched with the pump of free running diode-pumped solid-state lasers.
In these architectures, high-power emission, beam shaping, and mode control are related to the extraction of symmetric and anti-symmetric resonant eigenmodes. In general, symmetric modes produce constructive interference in the far-ﬁeld and have efﬁcient outcoupling into free space. In contrast, anti-symmetric modes interfere destructively in the far-ﬁeld, but have lower loss. As such, anti-symmetric modes are favored for lasing, but con- sequently provide limited extraction power. A number of pho- tonic approaches have been explored to circumvent these intrinsic limitations, while still ensuring directional beam proﬁles. These include vertically emitting graded photonic hetero- structures 15,16 , quasi-periodic gratings 17 , and double-periodicity DFB gratings, engineered to achieve a simultaneous tailoring of the emission frequency and a tuning of the beam direction, via the independent control of the extraction and feedback wave- vectors 18 . Further approaches have utilized on-chip phased locked arrays 19,20 and metasurface reﬂectors, which comprise multiple cavities 21 , and induce directional THz QCL emission in pulsed operation.
single-frequency operation. SDLs typically consist of a multi- quantum-well (QW) gain region monolithically grown on top of a distributed Bragg reflector (DBR), which serves as the end mirror in an external laser resonator. Further, the extended cavity of the SDL enables efficient intracavity nonlinear conver- sion, which in turn extends spectral coverage to wavelengths where bandgap engineering is nontrivial. InGaAs QW SDLs are now well-established systems due to their high gain, high- quality AlGaAs DBRs, and suitability for pumping with efficient 808 nm diodes. Moreover, they can provide laser emission with tens of watts of output power and high beam quality in the region between ∼920 nm and 1180 nm by tuning the indium content in the wells [9,10]. Operation of SDLs at wavelengths beyond 1.2 μ m can be obtained with more challenging semiconductor epitaxy, such as the use of layers of InAs quantum dots (∼1.25 μm ) or by adding nitrogen to the InGaAs QWs (up to ∼1.55 μm [12,13]). Further, where monolithic growth on GaAs is no longer feasible, wafer fusion techniques have been developed that allow the fabrication of high-efficiency SDLs that emit directly in the eye-safe wavelength region (specifically at 1.3 μm, 1.48 μm, and 1.58 μm to date [14,15]). However, the complexity of such growth and fabrication has motivated further research into practical alternatives, such as the use of intracavity crystalline Raman media to Stokes-shift the emission wavelength of mature (commercialized), high-performance InGaAs SDLs [16 – 18], to close the gap towards monolithic GaSb-based SDLs (∼1.9−3 μm ). Thus, a single-platform semiconduc- tor laser technology could enable a broad range of applications from the blue (via second-harmonic generation) towards the mid-IR by changing commercially available intracavity optical components.
We demonstrate efficient surface-emitting terahertz frequency quantum cascade lasers with continuouswave output powers of 20–25 mW at 15 K and maximum operating temperatures of 80–85 K. The devices employ a resonant-phonon depopulation active region design with injector, and surface emission is realized using resonators based on graded photonic heterostructures (GPHs). GPHs can be regarded as energy wells for photons and have recently been implemented through grading the period of the photonic structure. In this paper, we show that it is possible to keep the period constant and grade instead the lateral metal coverage across the GPH. This strategy ensures spectrally single- mode operation across the whole laser dynamic range and represents an additional degree of freedom in the design of confining potentials for photons. V C 2014 AIP Publishing LLC.
Microdroplet DyeLasers Microdroplets, with their near atomically smooth surfaces and small sizes, offer an ideal optical cavity for making low threshold and compact dyelasers. In such microspherical cavities, light is confined by continuous total internal reflections at the liquid-air interface, and forms the so called whispering gallery modes (WGMs). Dye lasing has been demonstrated in free falling droplets , pendant droplets  and levitated droplets . Although the observation of dye lasing in droplets dates back to the 1970s , the first reported microdroplet dye laser in the literature was by Chang’s group . In this work, dye solution droplets were generated by a vibrating orifice, in which a cylindrical liquid jet passing through the orifice is induced to break up into equal-sized droplets by a mechanical vibration of the orifice with the proper frequency and amplitude . The main experimental results for 60µm diameter ethanol droplets containing Rhodamine 6G are shown in Figure 2.4. The droplets were pumped by 10ns laser pulses at 532nm. Lasing occurs only in the longer wavelength side of the emission spectrum due to the high self absorption at shorter wavelengths (Figure 2.4, right panel). The laser emission is confined near the surfaces of the droplets which is characteristic for WGMs (Figure 2.4 b). Pump threshold of ∼35W/cm 2 , which was three orders of magnitude
Such problems as described for the nitrogen pumped laser apply far less to xenon pumped systems, which have pulse lengths of the order of Ips. Early work on this type of laser was done by Hansch, Schawlow and Toschek C43, whose dye laser design was based on that for a cw system, with coaxial pumping and a 1mm dye cell at Brewster's angle in an astigmatically compensated three-mirror cavity. No tuning element was included, and it was Schearer [53 who obtained narrow linewidths (0.025nm) with a similar laser using off-axis pumping and a reflection grating for tuning purposes. Powers of around SOW in the dye output were obtained for a 1.6kW pump power. Gallardo et al C63 used an open stream dye laser, a commercial Spectra Physics Inc model, and obtained linewidths of 0.2nm, but with a pulse duration of only 80ns, more closely resembling the pulses from a nitrogen pumped dye laser than the microsecond
algorithm implements the constraint of a minimum edge-to-edge distance of 2 µm between each pair of holes to avoid overlap between the scatterers. An increasing number, N, of holes in a patterned surface of average side L leads to a correspondingly smaller aver- age inter-site distance a ¼ L = p ﬃﬃﬃﬃ N . The geometrical ﬁ ll- ing factor of our photonic patterns was de ﬁ ned as the ratio r/a in analogy with that of photonic quasi-crystal lasers 18,19 . We varied r/a in the range 5 – 34% to explore both the effect of different degrees of scattering on light propagation in the RLs and also the outcoupling to free space, resulting from the different optical con ﬁ nement of the main modes 17 – 19 . The corresponding ﬁlling fraction of our random resonators, determined by the area of the holes with respect to the gold-patterned resonator surface, was correspondingly varied in the range 2–36%. For r/a > 14%, a radius r = 8 µm was chosen to provide sufﬁcient light scattering and extraction. By contrast, for r/a ≤ 14%, the radius was set to r = 3 µm to reduce the ﬁ lling fraction while main- taining the inter-site distance always smaller than 100 µm so that there is a suf ﬁ cient number of holes (N > 100) from which the light can scatter. The disordered photonic structure was then surrounded by an irregu- larly shaped chromium layer deposited on the mesa border featuring protrusions of typical size ≈ 25 µm (see Fig. 1a). The Cr layer was engineered by drawing the protrusion vertices comprised within ﬁxed innermost and outmost limits to avoid overlap with the internal holes and so that its average width is kept comparable with the wavelengths of the expected lasing modes in the semiconductor (≈35 µm for 3 THz radiation). For resonators having different ﬁlling fractions but the same area, the Cr border was kept identical. The purpose of this partially absorbing border is to suppress undesired electromagnetic modes, such as whispering-gallery modes that have little overlap with the central random geometry; unlike the geometry proposed in ref. 42 , here such modes are inherently suppressed by design. To ensure a good balance between surface-related diffrac- tion and electric power dissipation, two sets of devices were designed with overall device areas (including the chromium-absorbing boundary) of 0.57 mm 2 (type A) and 0.70 mm 2 (type B). Figure 1a shows a scanning- electron-microscope image of a representative device.
The specifications of the modified Nd: YAG laser are now discussed. As has been stated above, the modified laser provided pulses of duration ~ 12 nanoseconds (FWHM) with a pulse energy of between 0.5 and 1.0 mJ at a wavelength of 532 nm. Examples of the temporal form of pulses produced by the laser are shown on figures 3(a) and 3(b). Evaluation of the frequency spectrum of the Q-switched pulse is now considered. For reasons that will shortly become appaient, in order to ascertain that SLM operation has been attained, measurements must be ctu*ried out simultaneously in both the frequency and time domains. Temporal pulse measurements were obtained using a 275 MFIz oscilloscope and fast (-3 0 GHz) photodiode giving an overall risetime of 1.3 nano seconds. A description of the pulse measurement system may be found in chapter VI. As can be seen by the figures the pulses may be smooth or show mode beating. As will be seen below the smooth pulses represent operation of the laser on a single longitudinal mode. The laser Iinewidth was measured as less than 300 MHz at (k = 532 nm) by using the 1.5 GHz free spectral range confocal interferometer which is described in chapter III of this thesis. However, the figure of 300 MHz represents an upper estimate of the laser Iinewidth since its represents the finesse of the interferometer at 532 nm. Since the intermode spacing of the laser is approximately 150 MHz this Iinewidth could represent oscillation on two adjacent modes. In this case the temporal profile of the laser can be used to establish the frequency spectrum of the pulse. If two adjacent modes are oscillating a beat wave between them is produced. The beat period is equal to the reciprocal of the intermode spacing, which for the 1.05 m cavity length, is around 6 ns. The temporal form of the optical pulse can therefore be used to obtain information about the pulse Iinewidth. Two modes oscillating with relative intensities I^ and I2, with difference frequency will have a temporal form which is given by,
Sixteen years later, in 1999 after a long period of heavy development and an unexpected convergence between ultrafast laser techniques and the ultranarrow continuouswave laser spec- troscopy, a new type of universal frequency chain was created in our laboratories by Udem et al. (1999b), Reichert et al. (1999) that could directly link an optical frequency to a radio frequency using only 5 different lasers where one was a so-called optical frequency comb generator based on a mode-locked titanium sapphire laser emitting a periodic pulse train with pulses of less than 100 fs duration. This universal tool was subsequently refined and improved by Reichert et al. (2000), Diddams et al. (2000) and Holzwarth et al. (2000) and received the name "optical fre- quency synthesizer"to express its universality for optical frequency metrology. This induced a revolution in optical frequency metrology as it then was possible to cover essentially the entire visible spectrum with such a device and even small laboratories could now reach accuracies of 10 −12 or better by simply using such a synthesizer together with a commercial rf primary fre- quency standard, a system that a single person could run and operate. Not only were many optical atomic transitions measured with this tool with previously unequaled accuracy (see for example in Niering et al. 2000, Udem et al. 2001, von Zanthier et al. 2000), but it also paved the way to reliable operation of atomic clocks using optical transitions (like in Diddams et al. 2001). These new optical atomic clocks hold the promise to improve frequency standards to even higher accuracies. Frequently it is predicted that they could reach a level of 10 −18 .
Abstract—Transmission loss measurements between a grid of hypothetical WSN node locations on the surface of a gas turbine engine are reported for eight frequencies at 1 GHz intervals in the frequency range 3.0 to 11.0 GHz. An empirical transmission loss model is derived from the measurements. The model is incorporated into an existing system channel model implemented using Simulink as part of a wider project concerning the development of WSNs for the testing and condition monitoring of gas turbine engines.
Abstract. A fully integrated planar sensor for 77 GHz automotive applications is presented. The frontend consists of a transceiver multichip module and an electronically steerable microstrip patch array. The antenna feed network is based on a modified Rotman-lens and connected to the array in a multilayer approach offering higher integration. Furthermore, the frontend comprises a phase lock loop to allow proper frequency-modulated continuouswave (FMCW) radar operation. The latest experimental results verify the functionality of this advanced frontend design featuring automatic cruise control, precrash sensing and cut-in detection. These promising radar measurements give reason to a detailed theoretical investigation of system performance. Employing commercially available MMIC various circuit topologies are compared based on signal-to- noise considerations. Different scenarios for both sequential and parallel lobing hint to more advanced sensor designs and better performance. These improvements strongly depend on the availability of suitable MMIC and reliable packaging technologies. Within our present approach possible future MMIC developments are already considered and, thus, can be easily adapted by the flexible frontend design.