JP2788054B2 - laser - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a laser, and more particularly to a dye laser.

BACKGROUND OF THE INVENTION Dye lasers are widely used as frequency tunable, almost monochromatic light sources. The spectral width of such a laser can be made almost monochromatic by positioning the laser to operate in a single longitudinal mode. A common way to achieve this narrow bandwidth single mode operation is to place some sort of frequency selective element in the laser resonator, such as a prism, diffraction grating, or interference filter. These devices typically introduce large losses, which reduce the Q of the cavity and require a sufficiently long cavity length to accommodate them. A major limitation of these devices is that frequency tuning becomes difficult once a single mode is set. This requires that the frequency selection element and the cavity length L be adjusted simultaneously. Because the frequency ν
Must satisfy the following mode conditions.

ν = mc / 2L where m is an integer and c is the speed of light. This is due to the demands on the accuracy and stability of mechanical parts.
Their size, position and motion must be controlled within half of the wavelength of the light. If these ranges are not met, the frequency becomes unstable and the tuning range becomes very tight.

SUMMARY OF THE INVENTION The present invention adapts a completely different method for producing tunable single-mode pulsed dye laser radiation.

SUMMARY OF THE INVENTION The device of the present invention uses a short, high-Q cavity that emits a longitudinal mode comb that spans the gain bandwidth of the dye. The selection of the single mode is made outside the laser cavity.

In accordance with the present invention, there is provided a resonator device for generating a plurality of modes of substantially monochromatic coherent radiation, and means external to the resonator device for selectively amplifying a predetermined one of the modes. An apparatus for generating a substantially monochromatic coherent radiation beam is provided.

[Example] Hereinafter, a description will be given with reference to the accompanying drawings.

In one embodiment of the present invention, a tunable single-mode pulsed dye laser uses superior means for producing monochromatic radiation. In this device, a short, high Q cavity emits longitudinal mode combs over the gain bandwidth of the dye. Single mode selection is made by a narrow band amplification system outside the cavity. The mode interval from the oscillator is Δνm <c / 2L. The bandwidth Δνamp of the amplification system is determined so as to satisfy the following condition.

Therefore, only one mode is effectively amplified and appears at the output according to Δνamp <Δνm. Equipment tuning is short cavity laser (SCL)
By adjusting the length L of the cavity. The design and construction of the device is simplified since no elements to be adjusted are present in the cavity. It is sufficient to simply adjust the center frequency of the narrowband amplifier (NBA) to include the mode in which the bandwidth Δνamp to be amplified is selected. Spectral width δ of any mode from short cavity laser SCL
Since ν is very narrow and δν << Δνm, it is relatively easy to ensure that this mode stays within the gain band of the narrowband amplifier NBA.

The tuning range of the device is determined by the change in the laser SCL length of the short cavity, ΔL, and is given by:

Δνrang = [ΔL / L] The principle of using a very short cavity to obtain single mode operation can be such that the mode spacing Δνm exceeds the bandwidth of the gain medium, for example in a diode laser or solid state laser. If you are well known. Such devices radiate in single mode from their resonant cavities. For dye lasers, such a method is not practical because the dye gain curve (FIG. 1a) has a very wide bandwidth.

The principle of using a short cavity laser SCL and a narrow band amplifier NBA to obtain tunable single mode operation depends on the design of both elements of the system (FIGS. 1b and 1c). Hereinafter, the configuration of one embodiment of the present invention will be described.

The short cavity laser SCL is shown schematically in FIG. It comprises mirrors M1 and M2 consisting of two plane or curved mirrors arranged in parallel at a distance L. The distance L is typically a few millimeters.

The mirror M1 is mounted on a stainless steel member 21 and is fixedly secured thereto by an O-ring seal 22. The mirror M2 is sealingly mounted on a stainless steel collar 23 mounted on a circular flexible diaphragm 24. The outer edge of the diaphragm 24 is attached to the stainless steel member 21 to seal the cavity formed between the mirrors M1 and M2. Two passages 25, 26 are provided in the member 21 to allow the dye solution to flow through the cavity.

A bent attachment member 27 of metal (eg, duralumin or steel) is firmly fixed to the stainless steel member 21 supporting the mirror M1. The mirror M2 is mounted on this mounting member 27 so that the mirror M2 can be adjusted parallel to the mirror M1. Mounting member for mirror M2
The connection to 27 is made by a rod or tube 28 of adjustable overall length. This is, for example, the piezoelectric transducer 29 (PZ
T). That is, the distance L between the mirrors M1 and M2
Can be adjusted by changing the length of the connecting rod or tube 28 supporting the mirror M2. Another method of adjustment is to change the pressure of the dye solution between the mirrors M1 and M2 to communicate the gas pressure on the outer surface of the diaphragm 24 or the mirror M2 to apply pressure to the connection of the diaphragm 24 or the bent mounting member 27. .

The reflectivity of the mirrors M1 and M2 is chosen to provide a high Q cavity for light at the laser wavelength of the dye used. Typically, the reflectivity of mirror M2 is close to 100% and the reflectivity of mirror M1 is 60-90% to act as an output coupler. Mirror M2 is also selected to have high transmission at the wavelength of the pumping radiation. This laser is a dichroic mirror, Mirror M2
Is excited by a beam from a pulsed laser (gas, solid, or liquid) coming in through. Alternatively, the mirror M1 may be configured as a dichroic mirror (having a high transmission at the wavelength of the pump radiation), so that the light of the pump can be incident on the dye. The laser output from the mirror M1 propagates in a direction opposite to the direction in which the pumping light enters, and can be separated from the pumping light by a dichroic beam separator. These pumping methods are shown in FIGS. 3a and 3b. The laser output from mirror M1 is focused by lens L1 and coupled to narrow band amplifier NBA.

The narrow band amplifier NBA is shown schematically in FIGS. 4a and 4b. It is composed of a transversely pumped dye cell 41 excited by a plurality of laser beams 42 to provide a series of parallel amplifying medium regions 1,2,3,4. Excitation is typically provided by the remainder of the pulse used to excite the short cavity laser SCL. The parallel area is obtained by splitting the pump beam by a polygonal prism followed by a cylindrical focusing lens. This parallel amplification region arrangement is used to form a compact system for performing traveling wave amplification and spectral filtering.

The output of the short cavity laser SCL is directed through the amplification region by a beam collimating mirror. The amplified output is spectrally filtered through a spectral filter for the dye cell to pass through the amplification region 2 before being reflected back at the roof-shaped prism P1. The beam is then reflected by top roof-shaped prism P2 into region 3 and so on. This series of successive amplification and spectral filtering operations is performed until the required bandwidth Δνamp is obtained. Two plane mirrors arranged at right angles can be used instead of the prisms P1 and P2.

The spectral filter may be a series of prisms, one or more interference filters, or a diffraction grating 43. A diffraction grating 43 is used in this embodiment. This is because the bandwidth of the device can be adjusted by changing the number N of grating grooves used. This is done by tilting the grating so that it spreads across different widths of the grating when the incident beam is at nearly parallel angles of incidence.

The center frequency of the passband of the narrowband amplifier NBA is prism P
Selected by rotating one. This is the angle φ determined by the wavelength λ and the incident angle θ according to the following equation:
Is set to reflect light diffracted from the grating.

λ = d (sin θ + sin φ) where d is the interval between the grating grooves. The single passband Δν limited by the diffraction of the device is given by:

The slope of the grating, and therefore N is the multiple pass bandwidth Δνamp
Is adjusted to ensure that is small enough to separate the single mode from the short cavity laser SCL. After traveling sufficiently around the narrowband amplifier NBA, the beam is reflected by a small mirror or prism and emitted out of the system. Since no resonant cavity is included in the narrowband amplifier NBA, there are no mode matching conditions to be satisfied and no requirements on mechanical tolerances.

To tune the frequency of the single mode output, the short cavity laser SCL is tuned by adjusting the voltage supplied to the PZT to move mirror M2. At the same time, the prism P1 in the narrowband amplifier NBA is rotated to maintain the center frequency of the amplifier band at the selected mode frequency. Prism P1 may be rotated by a sine bar drive or a rotary stage biased by PZT. In the narrow-band amplifier NBA, the voltage-graded and short-cavity laser SCL converter is adjusted to perform synchronous scanning.

The narrow width Δνamp can be adjusted to obtain the slave lock of the narrowband amplifier NBA so that one adjacent mode on either side of the central mode νm can be amplified.
This is shown in FIG. These modes νm-1
And νm + 1 are diffracted at different angles with respect to the central mode, thus producing an output that propagates at a small angle with respect to the central mode. Thus, they are spatially separated from the desired mode νm by a simple mirror system 63, 64 as shown in FIG. In this device, the output of the short cavity laser SCL61 passes through a narrow band amplifier NBA62. Therefore, the output consists only of the central mode νm. The intensity of this side mode is equal to νm centered on the peak of the band of the narrowband amplifier NBA. Therefore, their intensities can be used to generate an error signal to servo the center frequency of the narrowband amplifier NBA. Modes νm−1 and ν reflected by mirrors 63 and 64
The intensity of m + 1 is detected by the photodiodes 65 and 66, and the output signal is supplied to an electronic comparator (differential amplifier) 67. If an intensity difference is detected, a comparator
The output of 67 is amplified and used to drive servo controller 68. The servo controller 68 makes adjustments to the prism (not shown) to reset to an angle that gives equal intensity.

Scanning of the short cavity laser SCL results in a shift of νm across the band of the narrowband amplifier NBA resulting in a change in the relative intensities of the modes νm-1 and νm + 1. This change is detected by the above-described method using a comparator, and the setting of the narrow band amplifier NBA is adjusted to automatically compensate. Thus, the narrowband amplifier NBA is automatically driven to maintain its band around the single mode νm,
Therefore, it follows the tuning of the short cavity laser SCL.

The absolute frequency νm of the output of the system can be stabilized in the following way. Short cavity laser SC
L acts as a Fabry-Perot etalon with an interval L that can be monitored interferometrically. Short cavity laser
The SCL is shown as a monochromatic, continuously operating light source, for example a HeNe laser, and the resulting interference fringes are detected photoelectrically. A change in L causes a shift of these fringes and thus an error signal at the detector. Therefore, the servo control system can be locked to one side or peak of these fringes. Detection of fringe shift is used to energize the servo system to adjust L by changing the voltage applied to the PZT in the short cavity laser SCL. Therefore, the change in the photoelectric thickness L is compensated by the PZT, and the mode frequency νm is kept constant.
In addition, the error signal can be used to inhibit the ignition of the pumping laser, hence the short cavity laser
The SCL can receive a pumping pulse only when the length L is within a preset tolerance.

[Brief description of the drawings]

FIGS. 1a to 1c are characteristic diagrams showing the relationship between radiation intensity and frequency used to explain the operation of one embodiment of the present invention. FIG. 2 is a schematic diagram of a short cavity dye laser. 3a and 3b show an arrangement for pumping the laser of FIG. 4a and 4b are a plan view and a side view of the amplifier. FIG. 5 is a characteristic diagram showing an example of the relationship between intensity and frequency. FIG. 6 is a schematic diagram showing the layout of the apparatus according to one embodiment of the present invention. M1, M2: Mirror, 21: Member, 22: O-ring, 23 ...
Collar, 24 ... Diaphragm, 25, 26 ... Passage, P1, P2 ...
... Prism, 43 ... Diffraction grating.

Claims (13)

(57) [Claims] 1. A resonance device for generating radiation of a plurality of modes, and an interval between successive ones of the plurality of modes for selectively amplifying a predetermined mode among the modes. A device for generating substantially monochromatic coherent radiation having a narrow bandwidth and having narrow band amplifying means disposed outside said resonator, comprising: a series of parallel regions (1,2, A dye cell (41), in which a narrow band amplification system (NBA) is excited by multiple laser beams (42) and transversely pumped to provide (3,4)
An apparatus for generating substantially monochromatic coherent radiation, comprising: 2. The substantially monochromatic device according to claim 1, wherein said resonator comprises a cavity between two mirrors, the cavity configured to allow a dye solution to flow therethrough. A device that generates coherent radiation. 3. A substantially monochromatic coherent radiation according to claim 2, further comprising tuning means for tuning said resonator by adjusting the spacing between said two mirrors. apparatus. 4. An apparatus for generating substantially monochromatic coherent radiation according to claim 3, wherein said tuning means comprises a piezoelectric transducer mounted on one of said mirrors. 5. The substantially monochromatic coherent radiation of claim 3, wherein said tuning means comprises means responsive to a change in pressure in the dye solution in the cavity between the two mirrors. The device that generates. 6. Apparatus for generating substantially monochromatic coherent radiation according to claim 2, wherein the distance between said two mirrors is of the order of a few millimeters. 7. The method of claim 1, further comprising beam splitting means for deriving a plurality of laser beams from the pulses used to excite the laser. apparatus. 8. An apparatus for generating substantially monochromatic coherent radiation according to claim 7, wherein said beam splitting means comprises a polygonal prism. 9. A substantially monochromatic coherent radiation according to claim 1, further comprising spectral filtering means for continuously filtering radiation passing through said amplification medium. The device that generates the. 10. Apparatus for generating substantially monochromatic coherent radiation according to claim 9, wherein said spectral filter means is a diffraction grating. 11. The substantially monochromatic coherent radiation of claim 10, wherein said spectral filter means comprises tilt means for tilting a diffraction grating to adjust the pass bandwidth of said amplification system. The device that generates the. 12. The apparatus for generating substantially monochromatic coherent radiation according to claim 1, further comprising frequency selection means for selecting the center frequency of the passband of the narrowband amplification means. 13. An adjacent mode detecting means for detecting an amplitude of a mode adjacent to a center passage mode of the amplification system;
13. The apparatus for generating substantially monochromatic coherent radiation according to claim 12, comprising feedback means for adjusting a setting of said tilt means in response to a relative amplitude of said mode.