GB2168525A - Dye laser - Google Patents

Abstract

The dye laser is constituted by an oscillator including a diffraction grating 20 obliquely disposed with respect to the optical axis of an output mirror 10 and a total reflection mirror 30 opposing the diffraction grating 20. The oscillation wavelength of the oscillator is swept with time to produce a broadband oscillation spectrum having a uniform laser light intensity. Furthermore, a stabilizing means 100 is provided for stabilizing the output light of the light oscillator, thereby improving the time stability of the oscillation spectrum. <IMAGE>

Description

SPECIFICATION Dye laser apparatus BACKGROUND OF THE INVENTION Field of the invention This invention releates to a dye laser apparatus capable of effecting an oscillation of broadband modes.

Description of the Prior Art A prior art dye laser apparatus capable of oscillating in broadband modes is constituted by a resonator, wherein a dye cell is disposed between a total reflection mirror 1 and a partially transmissive output mirror 2 as shown in Fig. 14, or a resonator wherein the total reflection mirror 1 is substituted by a diffraction grating 4 as shown in Fig. 15.

The prior art broadband dye laser apparatus including a resonator of the construction just described has a defect that the intensity of the output laser light varies greatly in the width of the oscillation wavelength as shown by the oscillation spectrum shown in Fig. 16, that is the intensity of the laser light varies depending upon the wavelength. Furthermore, there is another defect that the oscillation spectrum varies for respective oscillation periods, that is, a defect that the time stability of the spectrum waveform is not good.

For this reason, in the dye laser apparatus of the type described above, it has been impossible to obtain high accuracy measuring results when the laser apparatus is used for the quantitative measurement of concentration employing CARS (Coherent Antistokes Raman Spectroscopy) or used for measurements of a transient phenomenon which varies with time.

SUMMARY OF THE INVENTION It is an object of this invention to provide a broadband dye laser apparatus in which the laser light intensity distribution is uniform in an oscillation wavelength width.

Another object of this invention is to provide a broadband dye laser device capable of preventing variation in the oscillation spectrum in respective oscillation periods having a gentle oscillation spectrum waveform, in other words having a satisfactory time stability of the spectrum waveform.

According to this invention, these and other objects can be accomplished by providing a dye laser apparatus comprising an oscillator including an output mirror, a diffraction grating obliquely positioned with respect to the optical axis of the output mirror, and a total reflective mirror confronting the diffraction grating, and wavelength sweeping means for varying with time the oscillation wavelength of the oscillator. In a modification there is provided means for stabilizing the output of the light outputted from the oscillator.

BRIEF DESCRIPTION OF THE DRA WINGS In the accompanying drawings: Figure 1 is a diagrammatic representation of one embodiment of a dye laser device embodying the invention; Figure 2 shows the oscillation spectrum of the oscillator shown in Fig. 1; Figure 3 is a perspective view showing one example of an optical polarizer; Figure 4 is a graph showing the operation of the optical polarizer shown in Fig. 3; Figure 5 shows characteristic curves showing modes of waveform scannings in a case wherein the light polarizer shown in Fig. 4 is used; Figures 6, 7 ad 8 show other examples of the optical polarizer; Figure 9 is a characteristic curve showing a mode of wavelength sweeping where the light deflector shown in Fig. 8 is used;; Figure 10 is a characteristic curve showing a mode of wavelength sweeping of a case wherein the optical polarizers shown in Figs. 6 and 8 are combined; Figure 11 is a perspective view showing one example of means for electrically varying the mirror angle; Figure 12 is a perspective view showing one example of a diffraction grating capable of electrically varying the number of slits; Figure 13 is a characteristic curve showing one example of the stabilized laser oscillation spectrum; Figures 14 and 15 show constructions of prior art broadband dye laser apparatus; and Figure 16 is a characteristic curve showing the oscillation spectrum of the dye laser devices shown in Figs. 14 and 15.

DESCRIPTION OF THE PREFERRED EMBODI MENTS A preferred embodiment of the dye laser apparatus shown in Fig. 1 comprises an oscillator A including an output mirror 10 in the form of a partially transmissive mirror, a diffraction grating 20 positioned obliquely with respect to the optical axis of the output mirror 10, and a turning mirror 30 in the form of a total reflection mirror confronting the diffraction grating.

When a solution in a dye cell 40 disposed between the output mirror 10 and the diffraction grating 10 is excited by excitation light, a light oscillation is created between the output mirror 10 and the turning mirror 30 and a portion of the oscillation light is taken out through the output mirror 10. The oscillator A including the diffraction grating 20 of the obliquely impinging type oscillates in a narrow wavelength width, the oscillation spectrum thereof being illustrated in Fig. 2. The exciting light impinging upon the dye cell 40 is given by a pulse oscillation laser device such as an excimer laser device, and a xenon flash lamp, or the like. The exiting light is applied intermit tently to the dye cell 40 at a predetermined period with the result that the dye laser apparatus of this embodiment also oscillates at the predetermined period.

In this embodiment, for the purpose of sweeping the oscillation waveform of the oscillator, in other words, for the purpose of varying with time the oscillation wavelength, an optical polarizer 50 is interposed between the diffraction grating 20 and the dye cell 40.

Fig. 3 shows one example of the optical polarizer.

The optical polarizer 50A shown in Fig. 3 comprises a plurality of prisms 51 formed of crystal of potassium biphosphate (KDP) which are angled with their directions of axis (direction of the axis in the direction of thickness d) alternately inverted and operate as foilows.

When a uniform electric field is applied in the direction of thickness d of respective prisms, a state will be created in which prisms of a refraction index (no+S) are placed in a medium of a refraction index (not6). As a consequence, light impinging upon one end of the polarizer will be polarized by an angle AO at the other end. When the oscillation light impinging upon the diffraction grating 20 is polarized by the optical polariser 50A, the angle between the diffraction grating and the oscillation light varies so that the wavelength varies with the result that the oscillation wavelength of the oscillator A varies. As shown in Fig. 3, a high voltage generating circuit 60A is connected to the optical polarizer 50A to apply high frequency electric field to the prisms 51.Consequently, as shown in Fig. 5, the oscillation wavelength of the optical polarizer 50A varies with time. In other words, the oscillation frequency is continuously swept by AA, and the sweeping operation is repeated at a period of that of the voltage generation of the high voltage generating circuit 60A. Consequently, according to this embodiment an oscillation spectrum having a broad waveform of the wavelength width hA can be obtained.

In Fig. 5, dotted lines show the oscillation spectrum at a certain polarization angle.

The sweeping width AA is determined by the construction and shape of the optical polarizer 50A. More particularly, where the longitudinal length of the polarizer is L=100mm, thickness is d=5mm, height h=5mm, and the maximum voltage impressed upon the polarizer 50A is 10 KV and the diffraction grating 20 has a size of 1800 G/mm, a sweeping width of about M1Ocm 1 can be obtained.

Wavelength sweeping similar to that described above is also possible when an optical polarizer 50B as shown in Fig. 6 is used. The optical polarizer 50B is of the type called a four electrode type electrooptical polarizer capable of polarizing incident light in the x or y direction by linearly varying the intensity of the magnetic field in the direction x or y. Accordingly, by applying a high frequency voltage generated by the high voltage generating circuit 60B across commonly connected electrodes 53a, 53c and electrodes 53b and 53d, a wavelength sweeping can be made repeatedly in a manner similar to that shown in Fig.

5.

When this optical polarizer is made of Li TaO3, and has a dimension that its edge length at one end surface is D=4mm, and that its longitudinal length L=5Omm, the optical polarizer undergoes a polarization of 2X10-2 rad, when it is applied with a voltage of 10 kV. Consequently, when a diffraction grating 20 of 1800 G/mm is used, a sweeping width of about LU=SOcm-' can be obtained.

An acoustic optical polarizer 50C shown in Fig. 7 can be used in place of the optical polarizer 5 shown in Fig. 1. In the optical polarizer 50C, when a ultrasonic vibrator 54 comprising a piezo-electric thin film or the like is excited by a ultrasonic vibrator 60c, ultrasonic wave will propagate through an acoustic medium 55 made of a molybdate crystal or the like in a direction shown by an arrow.

When light is impinged upon the medium 55, the impinging light will be deflected by the nonuniform refraction index formed by the ultrasonic wave, the deflection angle being proportional to the vibration frequency of the ultrasonic vibrator 60C.

The vibrator 60C is constructed such that its vibration frequency varies periodically. As a consequence, the optical polarizer 50C can repeatedly sweep the wavelength in the same manner as in the polarizer of previous embodiments.

Each of the optical polarizers 50A, 50B and 50C shown in Fig. 3, 6 and 7 is of an electrooptical polarizer of analogue type, but a digital type optical polarizer 50D as shown in Fig. 8 can also be used.

Referring to Fig. 8, in the optical polarizer 50D, a polarizer 56 transmit light having a polarization characteristic in the x direction, electrooptical elements 57a and 57b transmit the incident light as it is when voltage is not applied, whereas when voltage is applied, they rotate by 90" the direction of polarization of the incident light, and birefraction substances 58a and 58b permit incident light to pass straightforwardly when the direction of polarization of the incident light is in the x direction, whereas the substances 58a and 58b deflect the incident light when its direction of polarization is in the direction of y.

suppose now that the oscillation width of the oscillator A is 10ns. Then, where the combination of voltages impressed upon the elemetns 57a and 57b is suitably varied at an interval of 2ns during the oscillation width, light will be sequentially outputted through light paths (a), (b), (c) and (d). More particularly, when voltage is not applied to the elements 57a and 57b, the light will pass through the light path (a), when voltage is applied only to element 57b, the light will pass through the light path (c), whereas when voltage is applied to both elements 57a and 57b, the light will be outputted through the light path (d).

In the deflector 50D, by the action of an electrooptical element 57c provided at the last stage, the direction of polarization of the light passing through respective light paths (a)-(d) is made to coincide with the polarization characteristic of the diffraction grating 2a.

A control circuit 60D is provided for selectively driving the electrooptical elements 57a, 57b and 57c so 15 light of a predetermined direction of polarization will be sequentially outputted through light paths (a)-(b). Of course, this operation is made repeatedly.

Since the lights sequentially outputted through light paths (a)-(b) have different angles of polarization, even when this optical polarizer 50D is used, the oscillation wavelength can be swept.

The number of the light paths can be increased by increasing the numbers of the electrooptical elements and of the birefraction substance. For example, where four electrooptical elements and three birefraction substances are combined, an optical polarizer can be formed in which 8 lights having different angles of polarization are sequentially outputted. Fig. 9 illustrates an oscillation spectrum of an oscillator utilizing such optical polarizer.

In Fig. 9, where the sweeping wave wavelength width is made to be tA to obtain an oscillation wavelength width of 50 cm 1, and where a diffraction grating 20 of 1800 G/mm is used, a polarization angle of about 1" is necessary. Accordingly, where a polarizer having 8 light paths is constructed, the angle between respective light paths is set to about 0.125 , and the oscillator is constructed such that its oscillation wavelength width will be about 6 cm It is also possible to use an optical polarizer comprising a combination of an analog optical polarizer 50A shown in Fig. 3, and a digital optical polarizer 50D shown in Fig. 8.More particularly, where the optical polarizer 50D is constructed to have 8 light paths for sequentially impinging 8 lights polarized by the optical polarizer 50D upon the optical polarizer 50A, the lights will be subjected to an analogue polarization by the optical polarizer 50A.Thus, when such combined optical polarizer is used, the oscillation wavelength will vary with time for the 8 lights as shown in Fig. 10 with the result that where the sweeping wavelength width caused by the analogue optical polarizer 50A is made to be 5 cm ', the sweeping wavelength width of the combined optical polarizer would become to about 40-50 cm Although in the foregoing embodiments, the oscillation wavelength was caused to vary with time by varying the incident angle of the light to the diffraction grating 20 with the optical polarizer 5, it is also possible to sweep the oscillation wavelength without using an optical polarizer.

More particularly, as shown in Fig. 11, a thin film mirror 71 is bonded to one side surface of a wedge shaped piezoelectric element 70 and when a high frequency voltage generated by an oscillation circuit 72 is applied to the piezoelectric element 70, it periodically displaces so that the angle of the mirror 71 varies with the same period. For this reason, when the mirror 71 is substituted for the turning mirror 30 shown in Fig. 1 to vary the angle, the oscillation wavelength can be swept.

In Fig. 12, a diffraction grating 80 is formed with slits by a ruler or holograph. Upon application of a high frequency voltage outputted from an oscillation circuit 81 to the diffraction grating, the grating expands or contracts in accordance with the variation in the period of this voltage so that the spacing between the.

slits or the number of the slits per unit length varies with time. accordingly, when this diffraction grating 80 is substituted for that 20 shown in Fig. 1 for periodically varying the spacing between the slits, the sweeping of the oscillation wavelength can also be made.

As shown in Figs. 5 and 9, the spectrum caused by the sweeping of the wavelength has a broad waveform, but as shown by these figures, the intensity of the dye laser pulse emitted from the output mirror 10 varies somewhat within the sweeping wavelength width hA. In other words, the waveform of the oscillation spectrum fluctuates slightly.

Furthermore, Figs. 5 and 9 illustrate the spectrum of a laser pulse, but there is a fear that the waveform of the oscillation spectrum of each laser pulse might be caused to vary by the fluctuation in the exciting light impinging upon the dye cell.

For this reason, in the embodiment shown in Fig. 1, an intensity stabilizing device 100 is disposed to the right of the output mirror 10 to stabilize the intensity of the laser light emitted from the mirror 10.

In the intensity stabilizing device 100, color laser light passed through a amplifying dye cell 90 is impinged upon an optical attenuator 103 made up of an electrooptical element 101 and a polarizer 102. The color laser light has a certain polarizing characteristic determined by the diffraction grating 20. When supplied with a 1/2 wavelength voltage, the electrooptical element 101 rotates by 90" the polarizing plane of the incident light. On the other hand, when the voltage is not applied, the polarizer 102 is positioned such that it coincides the polarization plane of the output light of the electrooptical element 101 with the polarization plane of the color laser light.Accordingly, when the polarization state of the laser light outputted by the element 101 is varied by varying the voltage applied to the electrooptical element 101, the intensity of the laser light outputted from the optical attenuator will vary.

The optical attenuator 103, beam splitter 104, photodetector 105, control circuit 106 and high voltage generating circuit 107 constitute a feedback system obtaining stable color laser light.

In more detail, a portion of the laser light passing through the beam splitter 104 in the form of a half-mirror is inputted to the photodetector 105. The control circuit 106 compares the output of the photodetector 105 with a predetermined reference voltage to apply to the high voltage generating circuit 107 a control signal corresponding to the difference between the compared voltages, whereby the voltage applied to the electrooptical element 101 is varied so as to make zero the difference between the compared voltages.

For this reason, when the reference voltage is preset to obtain a predetermined reference voltage shown in Fig. 13, it is possible to obtain color laser light free from any variation as shown in Fig. 13. Furthermore, since the problem that the intensity of the laser light varies at each oscillation period caused by the fluctuation of the exciting light impinging upon the dye cell 40, provision of the intensity stabilizing device enables to provide a stable oscillation spectrum. It should be understood that the intensity stabilizing device is not limited to that described above, and that any other means can be used so long as it can made flat the level of the laser light outputted by the oscillator.

Claims (9)

1. A dye laser apparatus comprising: an oscillator including a partially transmissive output mirror, a diffraction grating obliquely positioned with respect to an optical axis of said output mirror, and a total reflection mirror confronting said diffraction grating; and wavelength sweeping means for varying with time the oscillation wavelength of said oscillator.

2. The dye mirror according to claim 1 wherein said wavelength sweeping means comprises an optical polarizer disposed between said diffraction grating and said output mirror, and means for driving said optical polarizer such that light impinging upon said diffraction grating through said optical polarizer is polarized periodically by a definite angle.

3. The dye laser apparatus according to claim 2 wherein said optical polarizer comprises an electrooptical polarizer.

4. The dye laser apparatus according to claim 2 wherein said optical polarizer comprises an acoustic optical polarizer.

5. The dye laser apparatus according to claim 1 wherein said wavelength sweeping means comprises a piezoelectric element having a thin film mirror, and means for driving said piezoelectric element such that angle of said thin film mirror varies periodically.

6. The dye laser apparatus according to claim 1 wherein said wavelength sweeping means comprises a piezoelectric element provided with slits for diffraction on its surface, and means for driving said piezoelectric element such that spacing between said slits varies periodically.

7. The dye layer apparatus according to claim 1 further comprising means for stabilizing the intensity of laser light produced by said oscillator.

8. The dye laser apparatus according to claim 7 wherein said intensity stabilizing means comprises optical attenuating means disposed in a light path of laser light produced by said oscillator, and control means for controlling degree of attenuation such that intensity of said laser light passed through said optical attenuating means has a reference intensity.

9. The dye laser apparatus according to claim 7 wherein said optical attenuator comprises a polarizer for polarizing the laser light outputted from said oscillator and a polarizer for passing light polarized by said polarizer.