WO2003073567A1

WO2003073567A1 - A two color laser

Info

Publication number: WO2003073567A1
Application number: PCT/GB2003/000859
Authority: WO
Inventors: Robert Edward Miles; Mira Naftaly
Original assignee: The University Of Leeds
Priority date: 2002-02-27
Filing date: 2003-02-27
Publication date: 2003-09-04
Also published as: AU2003219274A1; GB0204531D0

Abstract

A two color laser comprising a laser cavity and a Fabry-Perot etalon, the laser cavity and Fabry-Perot etalon being optically coupled to form a coupled cavity resonator.

Description

A TWO COLOR LASER

The present invention relates to a two color laser and also to a device for generating terahertz radiation using such a laser. More particularly, but not exclusively the present invention relates to a two color laser comprising a Fabry-Perot interferometer located within the laser cavity.

Terahertz frequency (lTHz=10¹² Hz) sources have a number of applications in the fields of medical and industrial imaging and in telecommunications. The difficulty of generating teraherz radiation is well known, and is due to the rapid fall-off in the performance of electronics at frequencies above 0.3 THz and of optical sources below lOTHz.

Continuous-wave THz generation from two-color lasers has been demonstrated in a number of different laser systems. Although it is possible to generate CW THz radiation by photomixing two independent lasers, there are important advantages in using two- color lasers. In order to obtain stable-frequency narrow-band THz radiation the two laser sources must be independently stabilized to maintain a constant frequency difference, necessitating wavelength stability of a few GHz. In addition, the efficiency of the photomixing process depends critically on the spatial mode matching of the two laser beams, which requires them to be precisely collimated and aligned. Clearly, both frequency stability and alignment can be more easily achieved using a two-color laser.

Most reported schemes of two-color lasers for THz and microwave generation have concerntrated on laser diodes and microchip lasers, although an Er:Yb: glass laser and a Ti: sapphire laser have also been described. The two-color operation has been obtained by a variety of techniques: dual-mode DBR (distributed Bragg reflector) , external resonators with dual grating arrangements, and dual cavities with shared elements. However, all these schemes rely on external stabilization elements to maintain a constant THz frequency.

Accordingly, in a first aspect, the present invention provides a two color laser comprising a laser cavity and a Fabry-Perot etalon, the laser cavity and Fabry-Perot etalon being optically coupled to form a coupled cavity resonator.

The two color laser according to the invention has the advantage that no such external stabilisation elements are required.

Preferably, the coupled cavity resonator has at least two modes, two of the modes being separated by a frequency in the range 0.01 to 50THz, preferably 0.1 to lOTHz, more preferably 0.2 to 3THz.

The two color laser can comprise an adjustment means for adjusting the free spectral range of the Fabry-Perot etalon.

Preferably, the Fabry-Perot etalon comprises two parallel plates and the adjustment means is adapted to adjust the separation of the plates.

Alternatively, the Fabry-Perot etalon comprises an optical medium positioned between two parallel plates and the adjustment means is adapted to adjust the refractive index of the optical medium.

The parallel plates of the Fabry-Perot etalon can be glass plates. The glass plates can be coated to produce partial reflectivity. The spacing of the plates can be in the range 3μm to 15μm, preferably in the range 15μm to 1.5mm, more preferably in the range 50μm to 750μm.

The laser cavity can comprise at least one mirrow, the Fabry- Perot etalon being attached to the mirror. The laser cavity can comprise an active laser medium, the active laser medium comprising a rare earth doped glass. The rare earth coped glass can be one of a neodymium phosphate glass, on ytterbium doped silica glass or an erbium doped silica glass.

Alternatively, the laser can be a Ti-sapphire crystal laser. As a further alternative the laser can be a diode laser. As a further alternative the laser is a transition metal doped vibronic laser.

Rare-earth doped glass is the possible active medium for the two- color laser of the invention, due to the following advantages. The above mentioned rare-earth doped glass lasers have broad gain bandwidths, sufficient for the generation of terahertz radiation. Similar lasers using crystal hosts have much narrower gain bandwidths. These rare-earth doped glass lasers are pumped by widely available and highly reliable laser diode sources. As a consequences, a rare-earth doped glass laser can be more compact, have lower power requirements, and be more portable and robust than a vibronic crystal laser. In addition, where higher terahertz power is required, a second laser unit can be configured as an amplifier, to amplify both beams from the first unit before they are photomixed to generate terahertz radiation.

As mentioned above, the two-color laser of the invention may also be realised in a diode laser. A diode laser has the advantage of small size, robustness and direct electric excitations; however, the achievable power output is much lower than from doped glass or crystal lasers. The two-color laser of the invention can operate either in a continuous-wave (CW) or in a pulsed mode.

In a further aspect of the invention there is provided a device for generating terahertz radiation comprising a two color laser as claimed in any one of claims 1 to 11; and a photomixer for mixing laser radiation generated at two different frequencies by the two color laser to generate terahertz laser radiation.

Photomixing can be carried out using a non-linear optical material. Examples of such materials, which are known to operate well at visible/near-infrared wavelengths includes lithium- niobate, lithium tantalite, gallium selenide and zinc tellurite.

Photomixing can be carried out using a photoconductive antenna. Such antennas can be fabricated for example on a substrate of semi-insulating gallium arsenide or low-temperature-grown gallium arsenide .

The photomixer may be inserted into the laser resonator and will operate as an intracavity photomixer. This configuration has the advantage that radiation intensity inside the laser cavity is 10- 100 times higher than the intensity of the output beam. Consequently an intracavity photomixer will produce much more intense terahretz radiation, and will also have a higher conversion efficiency. In addition, placing the photomixer inside the laser cavity will produce a more compact and mechanically robust device.

The present invention will now be described by way of example only and not in any limitative sense with reference to the accompanying drawings in which

Figure 1 shows a device for generating THz radiation according to the invention. The device includes the two color laser according to the invention;

Figure 2 shows the output spectra of the two color laser of the embodiment of Figure 1 at different etalon separations; Figure 3 shows a measure of the Golay signal as a function of pattern position for the device of Figure 1; and,

Figure 4 shows the spectrum of a further two color laser according to the invention.

According to the present invention a laser is forced to oscillate at two wavelengths by inserting a Fabry-Perot etalon into the laser cavity. An intracavity Fabry-Perot etalon acts to create a coupled-cavity resonator, whereby only frequencies matching the etalon spacing are allowed to oscillate. The frequency difference between two color beams produced by a Fabry-Perot etalon is given by f=c/2nt , where f is the frequency difference, c is the speed of light, n is the refractive index of the etalon medium, and t is the etalon spacing. To produce a frequency difference in the terahertz range, the spacing (thickness) of the etalon must be of the order of lOOμ . The finesse of the etalon must be as high as possible, forcing the two color beams to have narrow linewidths, in order to generate near-monochromatic terahertz radiation. However, high etalon finesse reduces laser gain, and must therefore be limited to maintain high power laser operation.

The fluctuations in the THz frequency are greatly reduced owing to the common-mode noise rejection. The invention is easy to implement and can work successfully in many types of laser. The only requirement is that the laser line should have sufficient inhomogeneous broadening to support stable simultaneous oscillation of modes separated.

The laser used in the embodiment of the invention shown in Figure 1 is a Coherent RegA9000 Ti-sapphire regenerative amplifier set to run in CW mode. The pump source was 10 W Coherent Verdi-Vl-0, which is a frequency doubled Nd:YV0₄ diode-pumped laser. The CW laser power is 2.5 W, which drops to 2 W when an etalon is inserted into the cavity. The center wavelength varies between 790-800 nm and tends to drift over time. This however does not affect the mode pattern obtained from the laser. The resonator length was 1.8 m, giving a longitudinal mode spacing of 80 MHz.

An etalon for THz frequency generation must have a thickness of the order of ~0.1 mm. Microscope cover-slips of two different thicknesses are employed in this embodiment of the invention: these were 0.19+1 mm and 0.36+1 mm thick. Such glass cover-slips commonly have good optical properties, including flatness and parallelism. The cover-slips are uncoated, so that their reflectivity is due solely to the refractive index of the glass and is therefore

R = (n - l)² / (n + l)² * 0.04 , (1)

where n is the refractive index of glass and is ~ 1.5. The etalon finesse is then given by

F = π VR / ( 1 - R) ~ 0.65 . (2)

This finesse is sufficient to select out laser modes with appropriate frequency separations. Cavity modes matching the eltalon are always available because the cavity mode spacing is four orders of magnitude denser than that of the etalon. The etalon is held within the resonator in a mirror holder anchored to the optical bench externally to the laser, and therefore not stabilized with respect of the laser cavity. Nevertheless the mode pattern obtained from the laser maintains a constant frequency spacing, determined by the etalon. The laser modes are monitored by an optical spectrum analyzer (IST-REES E200) and are recorded by a digital oscilloscope.

Photomixing is performed by a large-aperture triangular antenna (shown in Fig. 1) fabricated on semi-insultating GaAs. The applied voltage is 150 V DC and the photocurrent is 3 mA. If the photocurrent is allowed to rise above 3 mA, thermal runaway is initiated, with the current increasing up to the supply limited and the THz signal disappearing. The radiation cone from this antenna is approximately 50°.

The THz radiation produced by this embodiment of the invention is detected by a Golay cell (QMC Instruments, Type OAD-7) sensitive from 10 cm^-1 to 650 cm^"1 (15-1000 μm) . The Golay cell had an aperture of 6mm and is placed directly behind the antenna at a distance of 5mm. In order to filter out the visible radiation from the laser, a Si wafer and a sheet of white printer paper are placed between the antenna and the Golay aperture. The signal from the Golay cell is detected by a lock-in amplifier (EG&G Instruments Model 7265) . An optical chopper is used to modulate the laser beam; the chopping frequency is 11Hz, and is selected to optimize the Golay signal and SNR.

Fig. 2 shows examples of the laser spectra obtained from the two color laser of figure 1 by inserting Fabry-Perot etalons into the laser cavity . The number of modes depends on the alignment of the resonator and of the etalon within it. Due to the mechanical instabilities in the setup, the number of modes and their relative strengths tended to vary spontaneously, changing gradually over a period of a few seconds.

Table 1 compares the calculated etalon frequencies with the modes observed in the laser spectra. The frequency difference (Δv) of modes produced by an etalon of thickness L is given by

Δv = c /2nL, (3)

which for a center wavelength λ correspond to a wavelength separation of Δλ = Δvλ² /c . ( 4 )

The mode structure of the observed laser spectra, as seen in Fig. 2 and Table 1, confirms that the mode frequency spectrum is due to the Fabry-Perot etalons in the cavity.

TABLE 1

MODES PRODUCED BY FABRY-PEROT ETALONS

Etalon thickness Etalon frequency and Observed wavelength wavelength difference difference

0.19Φ0.01 mm 0.53Φ0.03 THz

1.13Φ0.06 nm 1.20Φ0.02 nm

0.36Φ0.01 mm 0.28Φ0.01 THz 0.58Φ0.02 nm 0.60Φ0.02 nm

Photomixing of the laser modes is accomplished by a photoconductive antenna and the THz signal is detected by a Golay detector. The laser power falling on the antenna is ~1 W. The Golay signal is 0.1 mV, corresponding to an absorbed power of 2 nW. The conversion efficiency is therefore estimated to be ~10^~9.

In order to test the reliability and consistency of the system, a simple one-dimensional pattern is imaged. Fig. 3 plots the Golay signal overlayed on a schematic trace of the transmission pattern. To improve the SNR in this experiment the Golay signal was averaged over 1 min.

It is seen that the variation in the Golay signal was consistent with the transmission pattern. The spatial resolution islimited by the aperture of the Golay detector, which was 6mm.

In a further embodiment of the invention, a Ti-sapphire laser is used to demonstrate the two-color operation of a broadband laser with an intracavity Fabry-Perot etalon. Several etalson are used, consisting of thick glass slides coated on both sides with gold. Figure 4 shows examples of output spectra produced by the laser when the etalons were inserted into the resonator.

In another embodiment designed to demonstrate heterodyne photomixing from a two-color laser, the Fabry-Perot etalon consisted of two glass slides, each coated with gold on one side and separated from each other by 11cm. An avalanche photodiode was used to mix the beams and a spectrum analyser was used to detect the signal. This configuration produced a heterodyne signal at 1.4 Ghz.

Claims

1. A two color laser comprising a laser cavity and a Fabry- Perot etalon, the laser cavity and Fabry-Perot etalon being optically coupled to form a coupled cavity resonator.

2. A two color laser as claimed in claim 1, wherein the coupled cavity resonator has at least two modes, two of the modes being separated by a frequency in the range 0.01 to 50THz, preferably 0.1 to lOTHz, more preferably 0.2 to 3THz.

3. A two color laser as claimed in either of claims 1 or 2, further comprising an adjustment means for adjusting the free spectral range of the Fabry-Perot etalon.

4. A two color laser as claimed in claim 3, wherein the Fabry- Perot etalon comprises two parallel plates and the adjustment means is adapted to adjust the separation of the plates .

5. A two color laser as claimed in claim 3, wherein the Fabry- Perot etalon comprises an optical medium positioned between the two parallel plates and the adjustment means being adapted to adjust the refractive index of the optical medium.

6. A two color laser as claimed in any one of claims 1 to 5, wherein the laser cavity comprises at least one mirror, the Fabry-Perot etalon being attached to the mirror.

7. A two color laser as claimed in any one of claims 1 to 6, wherein the laser cavity comprises an active laser medium, the active laser medium comprising a rare earth doped glass . A two color laser as claimed in claims 7, wherein the rare earth doped glass is one of a neodymium phosphate glass, an ytterbium doped silica glass or an erbium doped silica glass .

A two color laser as claimed in any one of claims 1 to 6, wherein the laser is a Ti-sapphire crystal laser.

A two color laser as claimed in any one of claims 1 to 6, wherein the laser is a diode laser.

A two color laser as claimed in any one of claims 1 to 6, wherein the laser is a transition metal doped vibronic laser.

A device for generating terahertz radiation comprising a two color laser as claimed in any one of claims 1 to 11; and

a photomixer for mixing laser radiation generated at two different frequencies by the two color laser to generate terahertz laser radiation.

A device as claimed in claim 12, wherein the photomixer comprises a photoconductive antenna.

A device as claimed in claim 12, wherein the photomixer comprises a non-linear optical material.

A device as claimed in claim 14, wherein the non-linear optical material can include one of lithium-niobate, lithium tantalite, gallium selenide and zinc tellurite.