WO2014076444A1

WO2014076444A1 - Ultrafast semiconductor lasers as optical pump sources in terahertz systems

Info

Publication number: WO2014076444A1
Application number: PCT/GB2013/000486
Authority: WO
Inventors: Edik Rafailov; Ross LEYMAN; Natalia BAZIEVA
Original assignee: The University Of Dundee
Priority date: 2012-11-14
Filing date: 2013-11-14
Publication date: 2014-05-22
Also published as: GB201220460D0

Abstract

Improved laser sources for the generation of terahertz radiation which comprise a semiconductor diode laser for generating optical pulses for integration into a THz pulse generation system wherein the pulse generator comprises an ultrabroadly tunable quantum dot (QD) based laser diode (191) which acts as a single or dual-mode optical pump source. The QD is deposited in layers which may be in a range of physical sizes, dot densities and layer spacings. The external cavity may comprise a beam splitter (187) and two diffraction gratings (197) for individual control of the two wavelengths emitted via the opposite face of the QD gain chip (191). THz radiation is generated at the frequency difference of these two laser wavelengths.

Description

Ultrafast Semiconductor Lasers As Optical Pump Sources In Terahertz

Systems

Introduction

The present invention relates to the generation of electromagnetic radiation and in particular to improvements to the generation of electromagnetic radiation in the terahertz band of the electromagnetic spectrum. Background

Terahertz radiation is defined as electromagnetic radiation having a wavelength between the infra red region and the microwave region of the electromagnetic spectrum.

Quantitatively, this radiation band can be defined as being between 0.03mm and 3mm in wavelength, although it is known in the art to use broader and narrower wavelength bands when discussing terahertz radiation. One interesting property of terahertz radiation is its ability to penetrate

deep into many organic materials without the damage associated with ionizing radiation such as X-rays. Also, because terahertz radiation is readily absorbed by water, it can be used to distinguish between materials which contain different amounts of water. Such methods could allow effective detection of epithelial cancer with a safer and less invasive or painful system using imaging. Some frequencies of terahertz radiation can be used for 3D imaging of teeth and may be more accurate and safer than conventional X-ray imaging in dentistry.

Terahertz radiation can penetrate fabrics and plastics, so it can be used in surveillance, such as security screening, to uncover concealed weapons on a person. This is of particular relevance because many materials of interest have unique spectral "fingerprints" in the terahertz range. This offers the possibility of combining spectral identification with imaging. Spectroscopy in terahertz radiation may also provide novel information in chemistry and biochemistry. Methods of THz time-domain spectroscopy (THz TDS) and THz tomography have been shown to be able to perform measurements on, and obtain images of, samples which are opaque in the visible and near-infrared regions of the spectrum.

Many possible uses of terahertz sensing and imaging are proposed in

manufacturing, quality control, and process monitoring. These generally exploit the fact that plastics and cardboard are transparent to terahertz radiation, making it possible to inspect packaged goods.

Terahertz radiation may be generated by applying a laser pulse onto a

photoconductor device comprising a small piece of semiconductor crystal (commonly gallium arsenide) on which two planar metal electrodes form an antenna supporting a large electric field across its surface. The ultrafast (approximately 10Ofs) pulses of light from the laser (commonly from a titanium/sapphire laser at a wavelength of 800nm) are then focused onto the gap between the electrodes. This generates charge carriers in the form of electron-hole pairs and the application of a bias voltage accelerates these charge carriers, producing a THz pulse that is radiated.

Traditionally, the generation and detection of THz radiation is driven by ultrafast (sub-picosecond) optical signals emitted by TLSapphire and ring-dye lasers, for example. Sub-picosecond optical signals may be used here as the pulse widths in this range correspond to Fourier-transformed outputs in the 0.1-10 THz spectral region of interest, the so-called "THz gap", as well as significantly higher THz frequencies as the pump pulse width tends towards tens of femtoseconds and below.

Example applications of femtosecond optical pulses include the optical gating, or "pumping", of an ultrafast photoconductive (PC) switch as shown in Figure 1 a or electro optic (EO) crystal with significant nonlinear susceptibility in the pump wavelength range. Figure 1a shows a photoconductive switch 1 which provides a femotosecond optical pump pulse 3, micro antenna electrodes 5 and a bias voltage 7 which provides a THz output pulse 9. In the first case, a semiconductor material is fabricated that is characterised primarily by: charge carrier lifetimes below 1 ps; relatively high carrier mobility; an energy bandgap similar or lower than that of the pump energy; low dark current and a high avalanche breakdown threshold. This allows the material to electrically switch at the required THz rates, and the photoswitch electrodes are patterned as a geometrically appropriate sub-mm Hertzian dipole, for example. This fulfils the device's role as a radiative THz antenna, whereby the bias applied across the electrodes accelerates generated photocarriers and the current loaded through the switch over the ultrashort pump timescale is radiated as a THz electromagnetic pulse via the semiconductor substrate and into free space.

Figure 1 b, 1 c and 1d are graphs which plot the wave amplitude 13 against time in picoseconds 15 for input optical pulses of a first higher frequency 17 and a second lower frequency 9, the frequency and amplitude of the resultant THz output pluses graph 12 and their fourier transforms in the frequency domain in graph 14.

In the second case, an EO crystal such as UNBO₃ exhibits significant nonlinear. χ⁽²⁾ susceptibility is driven by intense, ultrashort optical pulses, which generate a nonlinear polarisation P⁽²⁾(t) within the active area that follows an intensity profile

The ultrafast optical pump is chosen with suitable wavelength (so as to avoid effects such as two-photon-absorption) and pulsewidth below 1 ps, as the intensity of THz electromagnetic fields emitted in this process follow the induced nonlinear polarisation timescale according to d²P&/dt². In this way, the nonlinear properties of the crystal inherently give rise to the generation of THz signals and pulsed THz radiation is generated "passively" in the system.

The drive to improve the efficiency and practicality of pulsed THz systems has in part led to the development of more compact and versatile ultrafast infrared and near- infrared laser systems, to eventually replace the bulky, power-consumptive, mechanically complex pump pulse sources such as the commonly-used Ti:Sapphire laser. These sources work well, but a more compact, efficient solution would greatly enhance the potential applicability and versatility of available THz systems and there have been significant developments in ultrafast semiconductor laser systems, for example, which make them almost ideal.

Low-cost, miniature semiconductor diode lasers are highly efficient light sources which have been widely used in a range of applications, most notably in optical fibre communications and biomedical applications. Improvements in their performance regarding ultrashort pulse generation, high beam quality, high peak and average powers are quickly making these devices extraordinarily interesting and promising candidates for an extended variety of applications in industry and science, ranging from nonlinear biomedical imaging, nano surgery, materials processing through to laser projector displays and free space optical communications. They are

exceptionally versatile devices, and may be engineered to operate over a range of optical wavelengths and output powers with considerable scope for tunability and manipulation of these parameters by alteration of both the structural and material design, as well as optical system configuration. Diode lasers may be designed with essential components such as the mirrored facets, gain medium, beam waveguide, electrical injection and distributed feedback wavelength tuning all monolithically integrated into the device, but may also be configured using external cavity setups. External cavity configuration allows a wide variety of optical wavelength tuning and pulse generation and/or compression techniques to be implemented, and many systems have been developed to allow diode lasers to perform with the required ultrashort pulse durations, output powers and spectral range and bandwidths to achieve levels competitive with established lasers like the Ti:Sapphire.

Figure 2 is an optical schematic diagram 31 of a high power, sub-picosecond diode laser based pulse system for use with pulsed THz Time Domain spectroscopy system (TDS). Figure 2 shows a diode laser 33 with a saturable absorber section 35. The output beam 34 of the laser following an optical path via a grating 37, lens 39, mirror 41 an isolator 43, a tapered amplifier 45, a mirror 4, a grating 49 and a mirror 51 to provide an output 53.

An example is shown in Figure 2, where a two-section diode laser 33 is passively mode-locked using the reverse-biased saturable absorber section 35 and set up in an external cavity using a diffraction grating 37 and an end mirror 41 as both external feedback and tuning elements. This output is then amplified using a tapered amplifier 45and the resultant high-power chirped output pulse is compressed using an external grating-based pulse compressor 44. This system allows a compact, diode laser system to operate at around 830 nm wavelength with a pulsewidth around 0.66 ps, average output power of 500 mW (2.5 kW peak) and a resultant achievable THz TDS bandwidth of 1.4 THz.

Optically-pumped semiconductor disk lasers (SDLs), also referred as vertical external cavity surface emitting lasers (VECSELs), are compact and versatile laser sources based on semiconductor gain materials. The parameters demonstrated by mode-locked SDLs to date include achieving <100 fs pulse duration, a broad range of repetition rates, >2 W average output power and peak power of 6.8 kW with potential applications ranging from optical communication to biophotonics. A semiconductor saturable absorber mirror (SESAM) is typically employed in the SDL setup to initiate mode-locking and shape the pulses. A typical SDL setup mode- locked by a SESAM is shown in Figure 3a. Figure 3a shows a laser 57 with a beam 59 which describes an optical path via a gain medium, a mirror 63 and a SESAM 65. Summary of the Invention

It is an object of the invention to provide improved laser sources for the generation of terahertz radiation. In accordance with a first aspect of the present invention there is provided

a semiconductor diode laser for generating optical pulses for integration into a THz pulse generation system wherein the pulse generator comprises an ultrabroadly tunable QD-based laser diode which acts as a single or dual-mode optical pump source.

Preferably, the QDs are deposited in layers.

Preferably, the layers may be of a range of physical sizes, dot densities and layer spacing. Preferably, the layers are deposited using molecular beam epitaxial techniques.

Preferably, pulse generator comprises a "chirped" gain bandwidth device

More preferably, the gain medium extends the gain bandwidth over around 200 nm in the 1.1-1.3 m range,

This aspect of the invention is tunable over a very wide spectral range at long (near telecoms-range) wavelengths, and utilises the inhomogeneous broadening of the QD gain medium to act as a highly efficient (low current threshold, low thermal sensitivity), ultra-compact, CW dual-mode optical pump source.

Preferably, the gain bandwidth is configured during growth of the chirped device thereby allowing an extra dimension of versatility when employing external cavity setups. One may design a chirped gain chip to operate as a semiconductor optical amplifier, with waveguide(s), electrical driving contacts and properties of the mirror facets all separately configurable. Preferably, the device further comprises highly anti-reflecting end facets and is operated in a double-Littrow configuration such that multiple longitudinal modes may be initiated and tuned in the setup whilst maintaining a single-beam for the multi- mode output using the opposite facet of the device. In accordance with a second aspect of the invention there is provided a

semiconductor diode laser for generating optical signals for integration into a THz pulse or CW generation system wherein the signal generator comprises a volume Bragg Grating (VBG) used in an external cavity configuration as a feedback element with a Quantum Dot Laser Diode to allow the generation of CW or ultrashort pulses of dual-wavelength light, for producing high peak power dual-mode pump signals to photoconductive and electro-optic (EO) THz emitters.

Preferably, the Quantum Dot Laser Diode is a two-section Quantum Dot Laser Diode. Advantageously, the spectral and temporal operational versatility of QD-based ultrafast LDs as discussed so far also allows scope for "hybrid" pump systems, which may take advantage of both pulsed and CW regimes to produce a highly efficient optical solution.

Single or Two-section tapered devices may be configured in an external cavity setup to provide an extra degree of output tunability, Preferably, the VBG is designed to reflect two specific wavelengths

Preferably, the VBG is a multiplexed VBG.

Preferably, the multiplexed VBG reflects two specific wavelengths when two gratings are 'interlaced' within the window.

Optionally, the multiplexed VBG reflects longitudinal wavelengths 1257±0.5nm and 1262±0.5nmw which having a wavelength separation of 5±0.1 nm, which

corresponds to a difference frequency of 1.078±0.021THz.

This VBG may be used in an external cavity configuration with a two-section QD LD to allow the generation of ultrashort pulses of dual-wavelength light, which provides a method for producing high peak power dual-mode pump signals to PC and EO THz emitters.

This is particularly advantageous in photoconductive antenna (PCA) devices as ultrashort pulses with high peak powers impart significantly lower thermal energy to the device, thereby reducing the risk of thermal breakdown, whilst still imparting very high optical pump energy and hence high THz output power. Thermal breakdown is a severe limitation in THz photomixer devices, and the THz output power of such devices scales as the square of the optical pump power. Some examples of a tapered two-section QD LD in this setup are given in Figure 8. Preferably, the VBG comprises customisable Bragg gratings which have been "implanted" throughout the volume of a small photothermorefractive (PTR) glass window using material doping and holographic writing techniques. PTR glass is a silicate glass doped with Ce, Ag, F and Br and is photosensitive. Under UV holographic exposure and a thermal development process, a refractive index change is induced in the window volume within the features defined by holographic pattern, which may be designed as successive planes throughout the window to form a Bragg grating.

Preferably, the two section quantum dot laser diode comprises a two section tapered device

Preferably, the two section quantum dot laser diode comprises a tapered gain section and an absorber section.

Preferably, the absorber section is parallel to the tapered gain section.

The parallel waveguide acts as a spatial filter within the cavity while the tapered section uses the increasing width of the gain medium to amplify the optical power.

Optionally, the laser diode is fully index-guided,

Optionally, the laser diode is fully gain-guided

Optionally, the laser diode is a combination of gain-guided and index-guided.

Preferably, the laser diode incorporates a QD active gain medium into the laser structure.

Preferably, the QD gain medium allows a broad gain bandwidth in the 1-1.3 pm wavelength range and ultrafast carrier recovery time when integrated as an absorber in QD-based tapered. lasers. Preferably, the gain-guided two-section tapered devices provide high peak output powers exceeding 17 W and enable the generation of Fourier-limited 0.672 ps pulsewidth optical signal. Optionally, the two-section Quantum Dot Laser Diode comprises an ultrabroadly tunable QD-based laser diode. An ultra-broadly tunable QD-based laser diode which acts as a dual-mode CW optical pump source enabling the device to produce ultrashort pulses while operating over a very broad spectral range Preferably, the QDs are deposited in layers.

Preferably, pulse generator comprises a chirped" gain bandwidth device

More preferably, the gain medium extends the gain bandwidth over around 200 nm in the 1.1-1.3 μιη range,

In accordance with a third aspect of the invention there is provided a semiconductor- diode laser (SDL) for generating optical pulses for integration into a THz pulse generation system wherein the semiconductor-diode laser is a Kerr lens mode- locked vertical external cavity surface emitting lasers (VECSEL).

Preferably the SDL generates optical pulses having a period of less than 1.5 ps.

Preferably the SDL generates optical pulses having a period of around 0.93 ps.

Preferably, the pulse repetition rate is between 150MHz and 300MHz Preferably, the pulse repetition rate is 210 MHz Preferably, the average output power is between 1 W and 3W

Preferably, the average output power is 1.5 W. Optionally, the system can operate at a peak power of 6.8 kW.

Optionally the system can be designed for mode-locked operation at predetermined optical wavelengths. Advantageously, the system does not need a SESAM within the cavity, which simplifies the optical setup, removes a significantly lossy element and eliminates the traditional SESAM system limitations as well as the need of the design and growth of the saturable absorber. The system can operate at a peak power of around 7kW, and can potentially be designed for mode-locked operation at longer optical wavelengths.

Preferably, the VECSEL semiconductor structure may be grown by conventional metal organic chemical vapor deposition. Preferably, the VECSEL has a gain section comprising strained InGaAs quantum wells (QW) designed for emission at 980 nm and surrounded by GaAs barrier regions, which was grown on top of a distributed Bragg reflector (DBR).

Preferably, charge carriers were constrained to the gain region by an Alo.15Gao.85As window layer and the structure was capped with 5 nm GaAs to avoid oxidation.

The active region was resonant-type and had total thickness of 1.5 pm.

Preferably, the VECSEL was pumped by an 808 nm diode laser with 150 pm radius pump spot.

In accordance with a fourth aspect of the present invention there is provided a semiconductor-diode laser (SDL) for generating optical pulses for integration into a THz pulse generation system wherein, the pulse is generated by a laser diode incorporating a tapered gain section and an absorber section.

Preferably, the laser diode is a two section laser diode.

The absorber section functions as an ultrafast variable reflector and spatial filter within the cavity while the tapered section functions to amplify the optical power by virtue of the increasing width of the gain medium.

Preferably, the absorber section is substantially parallel to the tapered gain section.

Optionally, the laser diode is fully index-guided,

Optionally, the laser diode is fully gain-guided

Optionally, the laser diode is a combination of gain-guided and index-guided. Preferably, the laser diode incorporates a QD active gain medium into the laser structure.

Preferably, the QD gain medium allows a broad gain bandwidth in the 1-1.3 pm wavelength range and ultrafast carrier recovery time when integrated as an absorber in QD-based tapered lasers.

Preferably, the gain-guided two-section tapered devices provide high peak output powers exceeding 17 W and enable the generation of Fourier-limited 0.672 ps pulsewidth optical signal.

The high output powers and ultrashort pulsewidths of signals generated by these semiconductor lasers, in addition to their good beam quality and relatively long operational wavelength makes them ideal candidates as the driving mechanism in a wide range of THz pulse generation systems based on both increasingly relevant, telecoms-wavelength PC emitters as well as EO crystals.

Additionally, as these miniature devices are designed with the active components such as the absorber, spatial filtering waveguide, gain section and mirror facets monolithically integrated, the entire pump system is extremely compact, low power and need only include a few standard additional conditioning optics such as an optical isolator and coupling lenses. Brief Description of the Drawings

The present invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure 1a shows an example of a femotosecond-pulsed photoconductive THz antenna setup, figures 1b, 1c and 1d show the optical pulse output, terahertz output and Fourier Transform spectrum of the output respectively;

Figure 2 is an optical schematic diagram of a known high power, sub-picosecond diode laser-based pulse system for use with pulsed THz TDS systems;

Figure 3a shows a known type of semiconductor disk laser mode-locked system using a SESAM for ultrashort pulse initiation and figure 3b shows a SESAM-free mode-locked semiconductor disk laser in accordance with the invention;

Figure 4 is an autocorrelation trace of a mode-locked SDL indicating sub-picosecond pulse duration it also shows multiple RF spectrum harmonics corresponding to cavity round-trips; Figure 5 is a typical optical spectrum of a SESAM-free mode-locked VECSEL in accordance with the invention it also shows a typical trace of the photodiode signal measured using a fast oscilloscope. Figure 6 is an example design of a two-section tapered, fully gain-guided QD-based laser diode in accordance with the invention.

Figure 7a is a graph which shows ultrabroad gain bandwidth of a chirped QD-based laser diode tuned via an external cavity feedback setup, figure 7b is a graph which shows broadly separated simultaneous dual-mode operation using a double-Littrow configuration and figure 7c is an optical schematic diagram of the double-Littrow configuration; Figure 8a shows an example dual-mode optical spectrum and figure 8b shows a corresponding autocorrelation trace showing a pulsewidth of ~6.5 ps, using a tapered two-section QD-based laser diode in an external cavity setup with a multiplexed VBG; Figure 9a is a graph of power versus wavelength for a two-section chirped QD gain chi, figure 9b shows an optical schematic diagram of a two-section chirped QD gain chip observed by tuning the device in an external cavity Littrow configuration, figure 9c is a graph which shows the range of output pulse durations while operating at different longitudinal wavelengths and figure 9d is a graph which shows the corresponding peak output powers.

Detailed Description of the Drawings

In one aspect, the present invention describes, a novel way to mode-lock SDLs without using any physical saturable absorbers in the cavity. Advantageously, this means that the limitations of SESAMs may be avoided and high kW peak powers with sub-picosecond pulse durations may be achieved. Consequently, SDLs with such parameters can be used to drive various nonlinear processes. A schematic drawing of a SESAM-free mode-locked SDL is shown in Fig. 3b 63. Figure 3b 63 shows a laser source 65 which provides a beam 67, VECSEL 69 and Mirrors 71 , 73, 75, 77 and 81 , slit 79 and output 83. Kerr lens mode-locking was first experimentally demonstrated in 1991 on vibronic lasers and revolutionized the field of pulsed lasers, opening up new application areas. This effect is predominantly employed in solid-state lasers such as the

TLSapphire and has also been demonstrated in semiconductor superlattice-based quantum cascade lasers.

However, the present invention describes the use of a Kerr lens mode-locked VECSEL. In one example, it can generate stable 0.93 ps optical pulses at a repetition rate of 210 MHz and with 1.5 W average output power which is suitable for application with THz devices.

Advantageously, the system does not need a SESAM within the cavity, which simplifies the optical setup, removes a significantly lossy element and eliminates the traditional SESAM system limitations as well as the need of the design and growth of the saturable absorber.

The system can operate at a peak power of 6.8 kW, and can potentially be designed for mode-locked operation at longer optical wavelengths. The VECSEL semiconductor structure may be grown by metal organic chemical vapour deposition. The results presented here are based on a gain section comprising 10 strained InGaAs quantum wells (QW) designed for emission at 980 nm and surrounded by GaAs barrier regions, which was grown on top of a distributed Bragg reflector (DBR) containing 30 pairs of GaAs/AIAs quarter wavelength thick layers. Charge carriers were constrained to the gain region by an Alo.15Gao.85As window layer and the structure was capped with 5 nm GaAs to avoid oxidation. The active region was resonant-type and had total thickness of 1.5 μιτι. The VECSEL was pumped by an 808 nm diode laser with 150 pm radius pump spot. The schematic of the external configuration is shown in Figure 3 b.

Mode-locked operation may be initiated in two different configurations. Firstly, stable mode-locking was observed while shortening the length of one of the cavity's arms, L, until the cavity operated near its stability limit. In this configuration, output powers up to around 700 mW may be achieved with an optical pump with around 18 W power. This allows the generation of pulses with duration of ~1.5 ps at 200 MHz repetition rate. Secondly, stable mode-locked operation may also be achieved by operating the cavity in its stability region. In such a configuration, a slit is inserted near the output coupler in order to achieve stable mode-locked operation. Pulses as short as 930 fs may be achieved at 210 MHz repetition rate.

A typical autocorrelation trace fitted by a sech²(t) function is shown in Figure 4. The inset of this figure shows a typical RF frequency spectrum indicating fundamental repetition rate and several subsequent harmonics. Figure 4 shows a graph 91 which plots intensity 93 against time delay 95. The inset 97 plots intensity 99 against frequency 101. There is a good agreement between the plot of the sech2 (t) function 105 and the data 103.

Figure 5a shows a graph 111 which shows a typical optical spectrum for a SESAM free mode locked Vecsel. The graph 111 plots intensity 113 against wavelength 115. Curve 117 shows a central wavelength of 985mm. Figure 5b is a graph 119 which plots intensity 121 against time. The pulses 125 being equally spaced.The VECSEL operates at a centre wavelength of 985 nm with a FWHM of 1.4 nm, shown in Figure 5a, which indicates pulses are around 1.3 times transform limited. Figure 5b shows a typical photodiode signal trace measured by a fast oscilloscope. Pulses in the graph are equally spaced by a cavity round trip time, indicating a single pulse operation. The achieved pulse duration could be limited by the nature of the resonant-type gain chip and dispersion within the semiconductor material. Typical peak output powers of this system (~6.8 kW) are almost four times greater than currently existing spontaneously mode-locked VECSELs and more than 20 times greater than previously reported peak powers achievable directly from SESAM- mode-locked VECSELs

The present invention further provides for the generation of high power ultrashort pulses to make use of a two section diode design, incorporating a tapered gain section and a (usually parallel) absorber section, shown in Figure 6. The parallel waveguide acts as a spatial filter within the cavity while the tapered section uses the increasing width of the gain medium to amplify the optical power. Tapered lasers may be designed in three different ways: fully index-guided, fully gain-guided or a combination of both designs, and are established now as a reliable semiconductor device configuration for the generation of high power, ultrashort optical pulses.

An interesting development in laser diode technology was the incorporation of QD active gain medium into the laser structure. QDs are often referred to as artificial or "designer" atoms, as they offer a further dimension of configurability within laser diode systems. QD gain medium allows, amongst other things, a broad gain bandwidth in the 1-1.3 μητι wavelength range and ultrafast carrier recovery time when integrated as an absorber in QD-based tapered lasers, making them ideal candidates for the generation of ultrashort pulses.

Gain-guided two-section tapered devices like these have demonstrated high peak output powers exceeding 17 W and enable the generation of Fourier-limited 0.672 ps pulsewidth optical signals. The high output powers and ultrashort pulsewidths of signals generated by these semiconductor lasers, in addition to their good beam quality and relatively long operational wavelength makes them ideal candidates as the driving mechanism in a wide range of THz pulse generation systems based on both increasingly relevant, telecoms-wavelength PC emitters as well as EO crystals. Additionally, as these miniature devices are designed with the active components such as the absorber, spatial filtering waveguide, gain section and mirror facets monolithically integrated, the entire pump system is extremely compact, low power and need only include a few standard additional conditioning optics such as an optical isolator and coupling lenses.

CW THz optoelectronic devices and materials have seen considerable interest and development since the first conclusive demonstration by E. R. Brown et al. of the THz photomixer in 1993. Laser-pumped CW THz systems are predominantly based on active elements such as electro optic (EO) crystals, or photoconductive (PC) materials in which (photo)carrier lifetimes are limited to 1 ps or shorter. PC and EO material(s) may be optically pumped by a signal composed of two distinct, narrow linewidth modes with a few nm wavelength difference. This pump signal may be set up by the spatial combination of two coherent optical beams, or by using a single beam generated by a coherent multi-mode source. A THz signal is the resultant difference frequency and, in the case of the PC antenna, the absorptive ultrafast material is modulated at this frequency and electron-hole pairs are generated and accelerated by the electric field set up by an integrated metallic antenna. As the material is electrically switched, a d.c. potential applied across the photoconductive antenna (PCA) is coupled through the device substrate and into free space, analogously to a traditional Hertzian-type dipole antenna described in the pulsed systems section above. In the case of the EO crystal, the process of self-difference-frequency mixing of the two optical modes within the crystal by harnessing its inherent ("passive") second order nonlinear susceptibility allows the generation and propagation of the resultant THz beat waves.

In another aspect of the present invention, a system is provided with an ultrabroadly tunable QD-based laser diode as a dual-mode C optical pump source for THz systems. Configuration of the operational wavelengths, or gain bandwidth, of QD- based LDs is engineered in part by the semiconductor deposition/growth process during production of the device. Using standard molecular beam epitaxial

techniques, the material grower may deposit layers of QDs with a range of physical sizes, dot density and layer spacing, for example.

Figure 7a is a graph 141 which plots output power 143 against wavelength 145 for four sets of initial conditions namely 10°C, 20%OC) 147, (10°, W/O OC) 149, (30°, 20% OC) 151 and (30°, W/O OC) 153. The graph shows a higher, more constant output power over a wavelength range from approximately 1120mm to 1320mm.

Figure 7B is a graph 155 which shows broadly separated simultaneous dual mode operations using a double Littrow configuration. An offset value 57 is plotted against wavelength 159 for power and frequency values (115mW, 0.55 THz) 161 , (107mW, 2.9THz) 163 (105mW, 6.2THz) 165 (98mW, 8.96THz) 167 (75mW, 12.98THz) 169 and (36mW, 16.32THz) 171.

Figure 7c shows a dual mode double Littrow configuration 173 with graphs 177, 183 and 185 which represent the frequency output at different parts of the system showing multiple longitudinal modes. The set up comprises a beam splitter 187, an output to a beam dump or OSA 189, diffraction gratings 197, 199 and the chirped quantum dot gain chip 191 with associated optics.

The configuration of QD LD-based setups with gain medium m may be engineered to extend the gain bandwidth over around 200 nm in the 1.1-1.3 μηη range, in a so- called "chirped" gain bandwidth device as shown in Figure 7a. This method involves the deposition of multiple layers of QDs with a variety of dot dimensions, allowing a broad range of operational energies/wavelengths. This solution is a highly versatile one as it is operational and tunable over a very wide spectral range at long (near telecoms-range) wavelengths, and utilises the inhomogeneous broadening of the QD gain medium to act as a highly efficient (low current threshold, low thermal sensitivity), ultra-compact, CW dual-mode optical pump source.

Configuring the gain bandwidth of these devices during growth allows an extra dimension of versatility when employing external cavity setups. One may design a chirped gain chip to operate as a semiconductor optical amplifier, with waveguide(s), electrical driving contacts and properties of the mirror facets all separately

configurable. A device with highly anti-reflecting end facets may be operated in a double-Littrow configuration as shown in Figure 6(b) such that multiple longitudinal modes may be initiated and tuned in the setup whilst maintaining a single-beam for the multi-mode output using the opposite facet of the device. An example of the tunable (THz-range) difference frequency optical beam output from such a setup is shown in Figure 6(c). Additionally, these chirped gain chips may be produced as two- section devices for ultrashort pulse operation, as discussed in previous sections.

It is also possible to generate high power (several Watts) dual-mode optical signals from VECSEL devices such as those previously discussed, for similar application with PC and EO THz emitters. This is done in an external cavity configuration using an etalon to select to very narrow linewidth modes from a broader spectral output from the VECSEL. The spectral and temporal operational versatility of QD-based ultrafast LDs as described herein also allows scope for "hybrid" pump systems, which may take advantage of both of these elements to produce a highly efficient optica! solution. Two-section tapered devices such as the one shown in Figure 6 may be configured in an external cavity setup to provide an extra degree of output tunability, with the use of volume Bragg gratings (VBGs) as external feedback elements.

VBGs are essentially customisable Bragg gratings which are "implanted" throughout the volume of a small photothermorefractive (PTR) glass window using material doping and holographic writing techniques. PTR glass is a silicate glass doped with Ce, Ag, F and Br and is photosensitive. Under UV holographic exposure and a thermal development process, a refractive index change is induced in the window volume within the features defined by holographic pattern, which may be designed as successive planes throughout the window to form a Bragg grating. "Multiplexed" VBG's may be designed to reflect two specific wavelengths when two gratings are 'interlaced' within the window..

In one example, a multiplexed VBG reflects longitudinal wavelengths 1257±0.5nm and 1262±0.5nm. The resultant separation of wavelengths returned by this VBG is 5±0.1nm, which corresponds to a difference frequency of 1.078±0.021THz. This VBG may be used in an external cavity configuration with a two-section QD LD to allow the generation of ultrashort pulses of dual-wavelength light, which provides a method for producing high peak power dual-mode pump signals to PC and EO THz emitters. This is particularly advantageous in PCA devices as ultrashort pulses with high peak powers impart significantly lower thermal energy to the device, thereby reducing the risk of thermal breakdown, whilst still imparting very high optical pump energy and hence high THz output power. Thermal breakdown is a severe limitation in THz photomixer devices, and the THz output power of such devices scales as the square of the optical pump power. Some examples of a tapered two-section QD LD in this setup are given in Figure 8.

Figure 8 is a graph 201 which plots intensity 203 against wavelength 205 for an example dual-mode optical spectrum, an expanded view is shown in inset 207. Figure 8b is a graph which plots intensity 211 against time delay 213 for data 217 and further shows the fit of a Lorentzian curve 2 5 to the data.

Figure 9a is a graph 2 9 which plots power 221 against wavelength 233 and shows examples of gain bandwidth for a two section chirped QD gain chip observed by tuning the device in an external cavity Littrow configuration. The Littrow configuration 231 is shown in figure 9b. this figure shows a diffraction grating 233, a gain section 235, an absorber 237, lens 239, further optical elements 241 and various outputs 245. Figure 9c is a graph 251 which plots pulse duration in picoseconds against wavelength 255, The inset 257 expands one section of the curve at around 1225mm. Figure 9d is a graph 261 at power 263 plotted against wavelength 265.

In the embodiment of the invention shown in figure 9, broadly tunable chirped QD LD such as the devices described in the previous section may also be configured as two-section devices, enabling the device to produce ultrashort pulses while operating over a very broad spectral range. Figure 9(a) shows an exemplary gain bandwidth of such a device in a single ("traditional") Littrow configuration, indicating operation over the wavelength range 1.18-1.32 μιτι, Figure 9(c) shows the typical range of output pulse durations achieved over this tuning range, and Figure 9(d) shows the corresponding peak output powers over this range. This system in a double Littrow configuration as described in the previous section also allows for a further dimension of both pulsed and CW pump signal versatility.

In conclusion, we have described the basis of a novel method for generating stable high power, sub-picosecond optical pulses around 980 nm wavelength with average powers of 1.5 W (peak power 6.8 kW) using an efficient, SESAM-free mode-locked VECSEL system. We have also described the use of various broadly tunable and ultrashort pulse QD LD systems as novel, efficient pump sources for THz

optoelectronic systems.

Improvements and modifications may be incorporated herein without deviating from the scope of the invention.

Claims

1. A semiconductor-diode laser for generating optical pulses for integration into a THz pulse generation system the system comprising a pulse generator with an ultrabroadly tunable Quantum Dot -based laser diode which acts as a single or dual- mode optical pump source.

2. An apparatus as claimed in claim 1 wherein, the QDs are deposited in layers. 3. An apparatus as claimed in claim 2 wherein, the layers may be of a range of physical sizes, dot densities and layer spacing.

4. An apparatus as claimed in claims 1 to 3 wherein, the layers are deposited using molecular beam epitaxial techniques.

5. An apparatus as claimed in claims 1 to 4 wherein, the pulse generator comprises a "chirped" gain bandwidth device

6. An apparatus as claimed in claims 1 to 5 wherein the laser diode has a gain medium which extends the gain bandwidth over around 200 nm in the 1.1-1.3 μπι range.

7. An apparatus as claimed in claims 1 to 6 wherein the laser diode is tunable over a very wide spectral range at long (near telecoms-range) wavelengths, and utilises the inhomogeneous broadening of the QD gain medium to act as a highly efficient (low current threshold, low thermal sensitivity), ultra-compact, pulsed or CW dual-mode optical pump source.

8. An apparatus as claimed in claims 1 to 7 wherein, the gain bandwidth is configured during growth of the chirped device thereby allowing an extra dimension of versatility when employing external cavity setups.

9. An apparatus as claimed in claims 1 to 7, wherein the laser diode further comprises end facets which are highly anti-reflecting, the laser diode being operated in a double-Littrow configuration such that multiple longitudinal modes may be initiated and tuned in the setup whilst maintaining a single-beam for the multi-mode output using the opposite facet of the device. 10. A semiconductor diode laser for generating optical pulses for integration into a THz pulse generation system wherein the pulse generator comprises a volume Bragg Grating (VBG) used in an external cavity configuration as a feedback element with a Quantum Dot Laser Diode to allow the generation of ultrashort pulses of dual- wavelength light, for producing high peak power dual-mode pump signals to photoconductive and electro-optic (EO) THz emitters.

11. An apparatus as claimed in claim 10 wherein, the Quantum Dot Laser Diode is a two-section Quantum Dot Laser Diode. 12. An apparatus as claimed in claim 10 and claim 11 wherein, the spectral and temporal operational versatility of QD-based ultrafast LDs provides a hybrid pump system.

13. An apparatus as claimed in claim 2 wherein the two-section tapered device is configured in an external cavity setup to provide an extra degree of output tunability,

14. An apparatus as claimed in any of claims 10 to 13 wherein, the VBG is designed to reflect two specific wavelengths 5. An apparatus as claimed in any of claims 0 to 14 wherein, the VBG is a multiplexed VBG.

16. An apparatus as claimed in any of claims to 10 to 5 wherein, the multiplexed VBG reflects two specific wavelengths when two gratings are 'interlaced' within the window.

17. An apparatus as claimed in any of claims 10 to 16 wherein, the multiplexed VBG reflects longitudinal wavelengths 1257±0.5nm and 1262±0.5nmw which having a wavelength separation of 5±0.1nm, which corresponds to a difference frequency of 1.078±0.021THz.

18. An apparatus as claimed in any of claims 10 to 17 wherein, the VBG is used in an external cavity configuration with a two-section QD LD to allow the generation of ultrashort pulses of dual-wavelength light, which provides a method for producing high peak power dual-mode pump signals to PC and EO THz emitters.

19. An apparatus as claimed in any of claims 10 to 18 wherein, the VBG comprises a customisable Bragg gratings contained in a volume of a small photothermorefractive (PTR) glass window.

20. An apparatus as claimed in any of claims 10 to 19 wherein, the quantum dot laser diode comprises a two section tapered device

21. An apparatus as claimed in claim 20 wherein the laser diode is a two section laser diode.

22. An apparatus as claimed in claim 20 or claim 21 wherein the absorber functions as an ultrafast saturable absorber and spatial filter within the cavity while the tapered section functions to amplify the optical power by virtue of the increasing width of the gain medium.

23. An apparatus as claimed in claims 20 to 22 wherein, the absorber is parallel to the tapered gain section and acts as a spatial filter within the cavity while the tapered section uses the increasing width of the gain medium to amplify the optical power.

24. An apparatus as claimed in claims 20 to 23 wherein, the laser diode is fully index-guided.

25. An apparatus as claimed in claims 20 to 24 wherein, the laser diode is fully gain-guided .

26. An apparatus as claimed in claims 20 to 24 wherein, the laser diode is a combination of gain-guided and index-guided.

27. An apparatus as claimed in claims 20 to 26 wherein, the laser diode incorporates a QD active gain medium into the laser structure.

28. An apparatus as claimed in claims 20 to 27 wherein, the QD gain medium allows a broad gain bandwidth in the 1-1.3 pm wavelength range and ultrafast carrier recovery time when integrated as an absorber in QD-based tapered lasers.

29. An apparatus as claimed in claims 20 to 28 wherein, the gain-guided two- section tapered devices provide high peak output powers exceeding 17 W and enable the generation of Fourier-limited 0.672 ps pulsewidth optical signal. 30. An apparatus as claimed in any of claims 10 to 17 wherein, the quantum dot laser diode comprises an ultrabroadly tunable Quantum Dot -based laser diode which acts as a dual-mode pulsed optical pump source.

31. An apparatus as claimed in claim 30 wherein, the QDs are deposited in layers.

32. An apparatus as claimed in claim 31 wherein, the layers may be of a range of physical sizes, dot densities and layer spacing. 33. An apparatus as claimed in claims 30 to 32 wherein, the layers are deposited using molecular beam epitaxial techniques.

34. An apparatus as claimed in claims 30 to 33 wherein, the pulse generator comprises a chirped" gain bandwidth device

35. An apparatus as claimed in claims 30 to 34 wherein the laser diode has a gain medium which extends the gain bandwidth over around 200 nm in the 1.1-1.3 pm range.

36. An apparatus as claimed in claims 30 to 35 wherein the laser diode is tunable over a very wide spectral range at long (near telecoms-range) wavelengths, and utilises the inhomogeneous broadening of the QD gain medium to act as a highly efficient (low current threshold, low thermal sensitivity), ultra-compact, pulsed or CW dual-mode optical pump source.

37. An apparatus as claimed in claims 30 to 35 wherein, the gain bandwidth is configured during growth of the chirped device thereby allowing an extra dimension of versatility when employing external cavity setups.

38. An apparatus as claimed in claims 30 to 35, wherein the laser diode further comprises end facets which are highly anti-reflecting, the laser diode being operated in a double-Littrow configuration such that multiple longitudinal modes may be initiated and tuned in the setup whilst maintaining a single-beam for the multi-mode output using the opposite facet of the device.

39. A semiconductor-disk laser (SDL) for generating optical pulses for integration into a THz pulse generation system wherein the semiconductor-diode laser is a Kerr lens mode-locked vertical external cavity surface emitting lasers VECSEL.

40. An apparatus as claimed in claim 39 wherein the SDL generates optical pulses having a period of less than 1.5 ps.

41. An apparatus as claimed in claim 39 or claim 40 wherein the SDL generates optical pulses having a period of around 0.93 ps.

42. An apparatus as claimed in claims 39 to 41 wherein, the pulse repetition rate is between 150MHz and 300MHz 43. An apparatus as claimed in claim 42 wherein, the pulse repetition rate is 2 0 MHz

44. An apparatus as claimed in claims 38 to 43wherein, the average output power is between 1W and 3W

45. An apparatus as claimed in claim 44 wherein, the average output power is 1.5W. 46. An apparatus as claimed in claims 39 to 45 wherein the peak power is around 7kW.

47. An apparatus as claimed in claims 39 to 46wherein, the system is mode- locked operation at predetermined optical wavelengths.

48. An apparatus as claimed in claims 39 to 46 wherein, the VECSEL has a gain section comprising strained InGaAs quantum wells (QW) designed for emission at 980 nm and surrounded by GaAs barrier regions, which was grown on top of a distributed Bragg reflector (DBR).

49. An apparatus as claimed in claims 39 to 48 wherein, charge carriers are constrained to the gain region by a window layer.

50. A semiconductor-diode laser for generating optical pulses for integration into a THz pulse generation system wherein the pulse is generated by a two section laser diode incorporating a tapered gain section and an absorber section.

5 . A diode laser as claimed in claim 50 wherein the diode laser is a two section diode laser.

52. An apparatus as claimed in claim 50 or claim 51 wherein the absorber functions as an ultrafast saturable absorber mirror and spatial filter within the cavity while the tapered section functions to amplify the optical power by virtue of the increasing width of the gain medium.

53. An apparatus as claimed in claims 50 to 52 wherein, the absorber is parallel to the tapered and curved gain section and acts as a spatial filter within the cavity while the tapered section uses the increasing width of the gain medium to amplify the optical power.

54. An apparatus as claimed in claims 50 to 53 wherein, the laser diode is fully index-guided. 55. An apparatus as claimed in claims 50 to 59 wherein, the laser diode is fully gain-guided .

56. An apparatus as claimed in claims 50 to 53 wherein, the laser diode is a combination of gain-guided and index-guided.

57. An apparatus as claimed in claims 50 to 56 wherein, the laser diode incorporates a QD active gain medium into the laser structure.

58. An apparatus as claimed in claims 50 to 57 wherein, the QD gain medium allows a broad gain bandwidth in the 1-1.3 pm wavelength range and ultrafast carrier recovery time when integrated as an absorber in QD-based tapered lasers.

59. An apparatus as claimed in claims 50 to 58 wherein, the gain-guided two- section tapered devices provide high peak output powers exceeding 17 W and enable the generation of Fourier-limited 0.672 ps pulsewidth optical signal.