Patent Description:
A Q-switch laser is a pulse laser configured to convert energy accumulated as a population inversion in a gain medium into an optical pulse having high energy by switching a Q factor of a laser resonator from low to high and to output the optical pulse. While the population inversion is being created in the gain medium in the resonator, the Q factor is maintained low in order to prevent laser oscillation and loss of the population inversion. When a sufficiently large population inversion is reached in the gain medium, the Q factor is increased to cause pulse oscillation.

A Q-switch carbon dioxide gas laser (for example, see <CIT>) using carbon dioxide gas as a gain medium, a solid state laser device (for example, see <CIT>) using a neodymium doped YAG crystal, and the like have been known.

It is also known that, upon incidence of pulse light on an optical resonator, when the incident light has a waveform indicating an exponential increase that matches a life of the resonator and has a pulse waveform that instantly becomes <NUM> at peak power, coupling efficiency to the resonator is maximized (for example, see<NPL>)).

Meanwhile, technologies of a continuous wave (CW) laser and an intermittent CW laser having high average power, such as a semiconductor laser and a fiber laser, have been developed. In particular, semiconductor lasers, being small and inexpensive, having low power consumption, and being configured to oscillate at a desired wavelength, have been widely used.

The document <CIT> describes a glass fiber laser system which includes a laser resonator cavity having a resonant path and an erbium-doped glass fiber lasing element with an output of from about <NUM> to about <NUM> micrometers within the laser resonator cavity. A light source directed into an input end of the glass fiber lasing element optically pumps the lasing element to emit light. A passive Q-switch lies along the resonant path within the laser resonator cavity. The Q-switch is formed of a host material having a concentration of uranium ions therein, so as to be a saturable absorber of the light emitted by the lasing element. The Q-switch is preferably a uranium-doped fluoride crystal such as U:CaF2, U:SrF2, or U:BaF2.

The document <CIT> relates to a laser arrangement (and a method of the laser arrangement, arranged to output energy in the form of laser emission, for emitting controlled Q-switched laser emission. The laser arrangement comprises a gain medium arranged to be excited when pumped, an optical resonator, an active Q-switch arranged in the optical resonator, said active Q-switch being controllable between at least a high loss state and a low loss state, and being arranged to introduce loss in the optical resonator to prevent lasing in the high loss state and to affect lasing minimally in the low loss state, a photo detector arranged to detect the presence of a free running pulse generated by the optical resonator and which occurs when a lasing threshold is reached and a processing circuitry arranged to control of the state of the active Q-switch based on the detection of the free running pulse.

The document <CIT> provides a passively Q-switched element or the like, which enables mode selection without increasing the number of components in a resonator in a Q-switched pulse laser or the like that oscillates in a great number of high-order modes and which is also applicable to a waveguide type laser in which a mode cannot be controlled spatially. By combining a saturable absorber with a transparent material which is transparent to a laser oscillation wavelength or the like, a passively Q-switched element having a mode selection function and a passively Q-switched laser device in which a passively Q-switched element has a mode selection function, and a planar waveguide type passively Q-switched element and passively Q-switched laser device are provided.

Finally, the document <CIT> describes a laser device which includes a resonator, an excitation energy supply part, a seed light source, a seed light introduction part, etc. The resonator includes a laser medium, a transparent medium, and an optical input/output part. When introduction of pulse seed light output from the seed light source into the resonator is set by the seed light introduction part, reproducing amplification operation is performed in which optically amplified pulse light is output by the optical input/output part to outside the resonator after pulse seed light introduced onto a resonance optical path by the optical input/output part is optically amplified by the laser medium. When non-introduction of the pulse seed light output from the seed light source into the resonator is set by the seed light introduction part, cavity dumping oscillation operation is performed in which the pulse light optically amplified by the laser medium is output by the optical input/output part to outside the resonator.

A laser with a short excitation life, such as a semiconductor laser, cannot sufficiently store energy and is not suitable for normal Q switch pulse oscillation. Also, in terms of light damage to a laser medium, it is difficult to extract an optical pulse having high peak power. An efficient pulse-extraction configuration is desired.

An object of the present invention is to provide a Q switch resonator that achieves efficient pulse extraction, and a pulse generator using the Q switch resonator.

An optical resonator is used separately from a laser medium, light is accumulated in the resonator, and a Q factor is switched from a high value to a low value to output an optical pulse.

In a first aspect of the present invention, a Q switch resonator includes an optical resonator formed of at least two mirrors, and configured to accumulate power of a continuous wave or an intermittent continuous wave incident from an outside, and a switching element provided in the optical resonator. When the power accumulated in the optical resonator increases to a predetermined level, the switching element outputs an optical pulse by lowering a Q factor from a first level to a second level lower than the first level.

In a second aspect of the present invention, a pulse generator includes a first optical resonator, and a light source configured to cause a continuous wave or an intermittent continuous wave to be incident on the first optical resonator. The first optical resonator includes a first switching element, and, when power of light incident on the first optical resonator increases to a predetermined level, the first switching element outputs an optical pulse by lowering a Q factor of the first optical resonator from a first level to a second level lower than the first level.

With the configuration described above, an optical pulse can be extracted efficiently.

In an embodiment, a pulse output having high peak power is generated by externally pulsing an output of a CW laser (including an intermittent CW laser). To achieve this, a resonator is provided separately from the laser, light is accumulated in the resonator, and a Q factor of the resonator is switched from a high value to a low value to output a pulse having high energy.

<FIG> is a basic configuration diagram of the present invention. A pulse generator <NUM> includes a CW laser <NUM> as a light source, and a Q switch resonator <NUM>. The CW laser <NUM> is configured to output a continuous wave or an intermittent continuous wave. The continuous wave or the intermittent continuous wave output from the CW laser <NUM> is incident on the Q switch resonator <NUM> and amplified.

The Q switch resonator <NUM> includes at least two mirrors <NUM> and <NUM>, and a Q switch <NUM> being a switching element. The mirror <NUM> and the mirror <NUM> form an optical resonator. Unlike a conventional Q switch laser, the Q switch resonator <NUM> does not contain a laser medium or a gain medium in the resonator.

The incident continuous wave or intermittent continuous wave from the outside is repeatedly reflected and amplified between the mirror <NUM> and the mirror <NUM>, and light energy is accumulated in the optical resonator. While the light energy is accumulated, a Q factor of the resonator is maintained high. When the power accumulated in the resonator becomes equal to or higher than a certain level, the Q factor is switched to a low side to release the accumulated power.

The Q factor is a dimensionless amount representing a continuous characteristic of oscillation of a resonant system, and is generally expressed in a ratio of an oscillation frequency to a line width (a full width half maximum (FWHM)) of a resonant frequency. The higher the Q factor, the more stably the system resonates, and the greater the energy therein.

When the energy in the resonator becomes sufficiently high, the Q switch <NUM> releases energy at once to reduce the Q factor. Thus, an optical pulse having high energy is output from the resonator.

This configuration is fundamentally different from a configuration of a general Q switch laser, i.e., a configuration in which a Q factor is maintained low while light energy is being accumulated in a gain medium, and the Q factor is increased for oscillation when the energy has been stored.

<FIG> are diagrams illustrating principles of the present invention. In <FIG>, CW light or intermittent CW light (hereinafter simply referred to as "CW light") incident on an optical resonator <NUM> from the outside is repeatedly reflected between the mirror <NUM> and the mirror <NUM> multiple times (S1). In <FIG>, an increase in the number of repetitions of reflection of the light between the mirror <NUM> and the mirror <NUM> causes an increase in energy accumulated in the optical resonator <NUM> (S2). In <FIG>, when the energy in the optical resonator <NUM> becomes sufficiently high, the Q factor is switched from high to low to release an optical pulse (S3).

<FIG> are graphs illustrating characteristics of the optical resonator <NUM>. Provided that the CW light is incident at time <NUM>, an optical pulse is output at time t1. In <FIG>, the Q factor is maintained high until the time t1. Accordingly, the light stably repeatedly reflects or oscillates in the optical resonator <NUM>. At the time <NUM>, the Q factor is switched to zero.

<FIG> shows a profile of power in the resonator. Energy is gradually accumulated in the resonator from the incidence of the CW light until the time t1. The origin corresponds to S1 in <FIG>, and the process of power increase corresponds to S2 in <FIG>. When the power reaches a constant level at the time t1, the power decreases at once by switching of the Q switch <NUM>. This point in time corresponds to S3 in <FIG>.

<FIG> shows a profile of output power. There is no output from the resonator until the time t1, and a sharp short pulse is output at the time t1.

In the basic configuration in <FIG>, a gain medium is not disposed in the resonator, and thus the problem of light damage is avoided. Sufficient energy can be accumulated in the optical resonator <NUM> by repeated reflection of the light between the mirror <NUM> and the mirror <NUM>, and an optical pulse having high peak power can be extracted.

<FIG> is a schematic diagram of the Q switch resonator <NUM> according to a first embodiment. The Q switch resonator <NUM> includes the mirror <NUM> and the mirror <NUM> that constitute the optical resonator <NUM>, and the Q switch <NUM>. At least one of the mirror <NUM> and the mirror <NUM> may be provided with an actuator <NUM> for adjusting a resonator length.

The mirror <NUM> and the mirror <NUM> are disposed such that light having made a round-trip in the resonator overlaps an original optical path. A power transmittance of the light when having made a round-trip in the resonator is ART.

The Q factor of the resonator is defined as Q = f/Δf= F/Lλ.

Herein, f represents a center frequency of a frequency spectrum, Δf represents an FWHM of the frequency spectrum, F represents a finesse, which is a resolution of the frequency spectrum, L is a length of a single round-trip of the resonator, and λ is a wavelength of the light.

The finesse F is defined as F = π (ART) <NUM>/[<NUM> - (ART)<NUM>].

The actuator <NUM> is used to set a resonator length in a single round-trip to be an integral multiple of the wavelength λ of the light. For example, a piezoelectric element or the like, for example, can be used for the actuator <NUM>.

The Q switch <NUM> is provided in the optical resonator <NUM>. The Q switch <NUM> is used to control the finesse F by changing an attenuation, i.e., a transmittance AQ of the light, and is also used to extract an optical pulse. Among characteristics of the Q switch <NUM>, a response speed τQ and a maximum transmittance AQMAX have great significance. The response speed τQ is preferably faster, and the maximum transmittance AQMAX is preferably higher.

<FIG> are configuration examples of the Q switch <NUM>. A Q switch resonator 10A in <FIG> uses a mechanical Q switch 15a. Example of the mechanical Q switch 15a include a rotatable mirror, with which a pulse is output to the outside of the resonator by mechanically changing an angle of the mirror. This configuration is used for an application that does not require a high response speed τQ (approximately milliseconds). The maximum transmittance AQMAX can be set equal to or greater than <NUM>% depending on an arrangement.

A Q switch resonator 10B in <FIG> uses an acoustic Q switch 15b. At a timing of a pulse output, an ultrasonic wave is applied to a crystal to excite a phonon of the crystal, thus causing light to be diffracted. While the response speed τQ is relatively fast (several-hundred nanoseconds), the maximum transmittance AQMAX is approximately <NUM>%.

A Q switch resonator 10C in <FIG> uses an electro-optical Q switch 15c. The electro-optical Q switch 15c includes, for example, a combination of an electro-optical crystal <NUM> and a polarizing beam splitter (PBS) <NUM>. A voltage, a current, an electric signal, or the like is applied to the electro-optical crystal <NUM> to rotate polarized light by using an electro-optical effect, and any one of polarization components is extracted by the PBS <NUM>. The response speed τQ is very fast (several nanoseconds), and the maximum transmittance AQMAX is approximately <NUM>%.

The Q switch <NUM> is not limited to these examples, and a magnetic body switch or the like may be used for the Q switch <NUM>. Further, when only the mirror <NUM> and the mirror <NUM> constitute an optical resonator, a Q switch may be obtained by setting either one of the mirrors to be variable in reflectance. A Q switch suitable for an application can be selected according to characteristics such as the response speed τQ and the maximum transmittance AQMAX.

<FIG> is a configuration example of a pulse generator 1A according to the first embodiment. The pulse generator 1A includes a CW laser <NUM> as an external light source, and a Q switch resonator 10D. In this example, the Q switch resonator 10D includes a triangular ring optical resonator 110D using three mirrors <NUM>, <NUM>, and <NUM>, and a Q switch <NUM> provided in the optical resonator 110D.

An optical system <NUM> for adjustment such that lateral modes of the CW laser <NUM> and the optical resonator 110D coincide with each other may be used. A feedback system <NUM> for coinciding a resonator length of the optical resonator 110D with an integral multiple of a wavelength of the CW laser <NUM> may also be used.

The CW laser <NUM> is a single longitudinal/lateral mode laser configured to perform continuous oscillation or pseudo-continuous oscillation (continuous oscillation time of micro seconds or longer). In one example, a semiconductor laser having a wavelength of <NUM> is used. An output beam of the CW laser <NUM> is assumed to be beam-shaped.

The optical system <NUM> for a lateral mode adjustment includes, for example, a mirror <NUM>, a mirror <NUM>, a lens <NUM>, and a lens <NUM>. Arrangements other than this example arrangement may be employed. By appropriately disposing an appropriate optical element, a lateral mode of laser light incident on the Q switch resonator 10D can be adjusted.

When the CW laser <NUM> operates in single lateral mode, an intensity distribution of light in a cross section orthogonal to an optical axis is a Gaussian distribution (TEM<NUM> mode). Both the intensity distribution (horizontal lateral mode) in a horizontal direction and the intensity distribution (vertical lateral mode) in a vertical direction of this cross section preferably coincides the lateral mode distribution of light resonating in the optical resonator 110D.

The lateral modes of the CW laser <NUM> and the optical resonator 110D may coincide with each other by appropriately designing and arranging the CW laser <NUM> and the optical resonator 110D without using the optical system <NUM> for a lateral mode adjustment.

The feedback system <NUM> configured to match a frequency of the CW laser <NUM> with a resonance frequency of the optical resonator 110D includes a photodetector <NUM> and a feedback circuit <NUM>. The photodetector <NUM> is configured to detect interference light of a portion of incident light reflected at the mirror <NUM> and a portion of light that makes a round-trip in the optical resonator 110D and to convert the detected lights into electric signals. The feedback circuit <NUM> specifies, from the electric signal, a difference between the frequency of the CW laser <NUM> and the resonance frequency of the optical resonator 110D, and feeds back the difference information to the optical resonator 110D or the CW laser <NUM>.

In the configuration example in <FIG>, difference information about a longitudinal mode is fed back to the actuator <NUM> of the optical resonator 110D. The actuator <NUM> is configured to perform a fine adjustment to a position of the mirror <NUM> to adjust a resonator length (a length of a round-trip of light). The difference information may be fed back to the CW laser <NUM> instead of the actuator <NUM>.

While a portion of light that makes a round-trip in the resonator is extracted from the mirror <NUM> and monitored by the photodetector <NUM> in the configuration in <FIG>, a portion of the light having made a round-trip may be extracted from the mirror <NUM> or the mirror <NUM> and compared with a longitudinal mode of incident light. In such a case, a reflectance of the mirrors <NUM>, <NUM>, and <NUM> that constitute the optical resonator is appropriately adjusted.

<FIG> are graphs illustrating resonator characteristics of the first embodiment. Parameters of a simulation are set as follows:.

In this condition, a round-trip resonator length (length of a round-trip) is an integral multiple of a laser wavelength. A calculation starts from a state in which power in the resonator is <NUM>.

<FIG> illustrates a transmittance AQ, <FIG> illustrates a Q factor of the resonator, <FIG> illustrates power in the resonator, and <FIG> illustrates a pulse output. An initial value of AQ is the maximum value AQMAX (AQMAX = <NUM>), and the Q factor is set high.

The power in the resonator gradually increases simultaneously with a start of incidence. When the power in the resonator approaches a steady value (approximately <NUM> microseconds), the AQ factor is lowered to <NUM> for a time of about <NUM> nanosecond to extract pulse light. The Q factor of the resonator and the power in the resonator decrease at once, and a pulse output is obtained.

<FIG> are enlarged views near <NUM> microseconds in <FIG>. Corresponding to <FIG>, <FIG> illustrates a transmittance AQ, <FIG> illustrates a Q factor of the resonator, <FIG> illustrates power in the resonator, and <FIG> illustrates a pulse output. Immediately after switching of the Q switch, i.e., immediately after switching of the AQ factor, a maximum pulse output of <NUM> W is obtained, and becomes a half value after about <NUM> nanoseconds from obtaining the maximum pulse output. The input power from the CW laser <NUM> is <NUM> W, so that the Q switch resonator <NUM> achieves a <NUM>-fold increase in peak power.

The peak power of the output pulse is proportional to the optical power (incident light × enhancement factor) amplified in the resonator. Therefore, the higher the power transmittance ART when the light has made a round trip in the resonator, the greater the peak power, but the power transmittance ART is limited by the maximum transmittance AQMAX. On the other hand, a pulse width is determined by time of round-trip of light in the resonator in a range in which a response speed of the Q switch is high. Then, an area of a pulse waveform represents output energy, and in the first embodiment, an optical pulse having high power and a short pulse width can be generated from the CW light by using the Q switch resonator <NUM>. An optical pulse having higher energy and a longer pulse width can be generated when using a longer resonator.

<FIG> is a schematic diagram illustrating a basic configuration of a pulse generator <NUM> according to a second embodiment. In the second embodiment, Q switch resonators are arranged in series, and an output pulse of a Q switch resonator <NUM> in a first stage is accumulated in a Q switch resonator <NUM> in a second stage for pulse compression to acquire higher peak power.

Similar to the first embodiment, the Q switch resonator <NUM> includes a mirror <NUM>, a mirror <NUM>, and a Q switch <NUM>. The mirror <NUM> and the mirror <NUM> are disposed such that light having made a round-trip in the resonator overlaps an original optical path.

The Q switch resonator <NUM> in the second stage includes a mirror <NUM>, a mirror <NUM>, and a Q switch <NUM>. The mirror <NUM> and the mirror <NUM> are disposed such that light having made a round-trip in the resonator overlaps an original optical path.

As described above, peak power of an output pulse is determined by input light and an enhancement factor. Then, the enhancement factor is limited by a maximum transmittance of the Q switch. Therefore, in order to obtain a higher peak power, the input light needs to be further increased. Thus, a pulse is compressed by generating a wide pulse in the Q switch resonator <NUM> in the first stage having a long resonator length, accumulating the first stage pulse in the Q switch resonator in the second stage having a short resonator length, and releasing the first stage pulse again.

Provided that the Q switch resonator <NUM> in the first stage has a round-trip length of L1 and the Q switch resonator <NUM> in the second stage has a round-trip length of L2, in a range where a switching speed of the Q switch is negligible, a compression rate of a pulse width is determined by a ratio L2/L1 of the round-trip lengths in the first stage and the second stage.

<FIG> is a configuration example of the pulse generator <NUM>. The pulse generator <NUM> includes a CW laser <NUM>, a Q switch resonator 10D in a first stage on which a CW wave output from the CW laser <NUM> is incident, and a Q switch resonator 20A in a second stage to which an output pulse of the Q switch resonator 10D is input.

In this example, the Q switch resonator 10D constitutes a triangular ring-type optical resonator with mirrors <NUM>, <NUM>, and <NUM>, and a Q switch <NUM> is provided in the optical resonator. Any of the mirrors <NUM>, <NUM>, and <NUM> may be provided with an actuator <NUM> for adjusting a round-trip length of the resonator.

The Q switch resonator 20A in the second stage constitutes a triangular ring-type optical resonator with mirrors <NUM>, <NUM>, and <NUM>, and a Q switch <NUM> is provided in the optical resonator. Any of the mirrors <NUM>, <NUM>, and <NUM> may be provided with an actuator <NUM> for adjusting a round-trip length of the resonator.

Output light of the CW laser <NUM> is input to the Q switch resonator 10D in the first stage by an optical system 30A for a lateral mode adjustment. The optical system 30A for a lateral mode adjustment includes, for example, a mirror <NUM>, a beam splitter <NUM>, a lens <NUM>, and a lens <NUM>. The beam splitter <NUM> guides a portion of incident light to the lens <NUM>, and guides another portion of the incident light to a mirror <NUM>. The light incident on the lens <NUM> passes through the lens <NUM> and is incident on the Q switch resonator 10D in the first stage with a lateral mode of the incident light being adjusted.

A resonator length of the Q switch resonator 10D is adjusted by a feedback system <NUM> to match with an integral multiple of a wavelength of the CW laser <NUM>.

An optical pulse output from the Q switch resonator 10D in the first stage is input to the Q switch resonator 20A in the second stage by an optical system <NUM> for a lateral mode adjustment. The optical system <NUM> is formed of a lens <NUM> and a mirror <NUM> as an example, but is not limited to this example. The optical pulse amplified and compressed by the Q switch resonator 20A in the second stage is extracted to the outside by a Q switch <NUM>.

A portion of the output of the CW laser <NUM> is guided by the mirror <NUM> to an optical system <NUM> for a longitudinal mode adjustment of the Q switch resonator 20A. The optical system <NUM> for a longitudinal mode adjustment includes, for example, mirrors <NUM> and <NUM> and lenses <NUM> and <NUM>, but is not limited to this example.

Light input from the optical system <NUM> for a longitudinal mode adjustment to the Q switch resonator 20A and reflected by the mirror <NUM> is monitored by a photodetector <NUM> of a feedback system <NUM>. More specifically, the photodetector <NUM> is configured to monitor interference light of the output light of the CW laser <NUM> guided by the optical system <NUM> and a portion of the light that makes a round-trip in the Q switch resonator 20A. The feedback circuit <NUM> is configured to control the actuator <NUM> or the CW laser <NUM> such that a resonator length of the Q switch resonator 20A matches an integral multiple of a wavelength of the CW laser <NUM>, based on information about the interference light.

<FIG> and <FIG> are graphs illustrating resonator characteristics of the second embodiment. <FIG> illustrate characteristics of a first stage resonator, and <FIG> illustrate characteristics of a second stage resonator. Parameters of a simulation are set as follows:.

A round-tip length of the first stage resonator and the second stage resonator is controlled to be an integral multiple of a wavelength of the CW laser <NUM>. A calculation starts from a state in which power in the resonator is <NUM>.

Switching of the Q switch <NUM> to extract an optical pulse from the Q switch resonator 10D in the first stage is preferably performed slower than that in the first embodiment. This will be described below.

<FIG> illustrates a transmittance AQ(<NUM>) of the Q switch <NUM> in the first stage, <FIG> illustrates a Q factor of the first stage resonator, <FIG> illustrates power in the first stage resonator, and <FIG> illustrates a pulse output of the first stage resonator. An initial value of AQ(<NUM>) of the Q switch <NUM> in the first stage is the maximum value AQMAX (AQMAX = <NUM>), and thus the Q factor is set high.

The power accumulated in the first stage resonator gradually increases from a start of incidence. When the power in the first stage resonator approaches a steady value, the AQ(<NUM>) value is lowered to <NUM> for a time of about <NUM> nanoseconds to extract pulse light in the first stage. The Q factor of the first stage resonator and the power in the resonator decrease slower than those in the first embodiment, and a pulse output in the first stage is obtained. A pulse width of the pulse output is wider than that in the first embodiment.

In <FIG>, an initial value of AQ(<NUM>) of the Q switch <NUM> in the second stage is the maximum value (<NUM>), and the Q factor is set high from the beginning. In <FIG>, the power in the second stage resonator rapidly rises immediately after an input of the first stage pulse. When the power in the second stage resonator reaches a peak, AQ(<NUM>) is lowered to <NUM> for a time of about <NUM> nanosecond to extract a pulse of interest. The power in the second stage resonator decreases at once, and the short pulse in <FIG> is acquired.

<FIG> and <FIG> are enlarged views near <NUM> microseconds in FIGS. lIA to 11D and <FIG>. Corresponding to <FIG> and <FIG>, <FIG> illustrates a transmittance AQ(<NUM>) in the first stage, <FIG> illustrates a Q factor of the first stage resonator, <FIG> illustrates power in the first stage resonator, and <FIG> illustrates a first stage pulse output.

The AQ(<NUM>) value in the first stage is lowered to <NUM> over a time of about <NUM> nanoseconds, and accordingly both the Q factor in the first stage and the power in the first stage resonator decrease slowly as compared to <FIG>. A pulse width of the first stage pulse output is about <NUM> nanoseconds. This value is longer than the pulse width of about <NUM> nanoseconds when the AQ(<NUM>) value is immediately set to <NUM>.

In <FIG>, the transmittance AQ(<NUM>) in the second stage is changed to <NUM> for a time of <NUM> nanosecond at a point in time when the power in the second stage resonator reaches a peak. The power in the second stage resonator decreases at once, and the short pulse in <FIG> is acquired. This short pulse is output at a maximum output of <NUM> W immediately after the Q switch <NUM> in the second stage is switched, and becomes a half value after about <NUM> nanoseconds from the output.

Thus, the output pulse of the Q switch resonator 10D in the first stage is input to the Q switch resonator 20A in the second stage, which allows for obtaining an ultra-short pulse having higher peak power.

<FIG> is a graph illustrating a Q factor switching speed of the first stage resonator. In general, upon incidence of pulse light on the optical resonator, coupling efficiency is <NUM>% when rise of a pulse waveform is an exponential function with a life (i.e., light confinement time) of the optical resonator as a time constant and instantly reaches <NUM> immediately after achieving peak power, coupling efficiency of <NUM>% is obtained.

In contrast, a pulse waveform emitted when the Q switch of the optical resonator in the first stage is instantly switched has a peak power achieved instantly from <NUM> and then exponentially falls with a life of the optical resonator in the first stage as a time constant (see <FIG>). This waveform has a shape that is obtained by inverting the ideal pulse shape described above in terms of time and changing the time constant. With such a pulse waveform, the coupling efficiency to the second stage optical resonator is insufficient.

Thus, as illustrated in <FIG>, by slowly lowering the transmittance AQ(<NUM>) in the first stage to <NUM>, rise of the pulse light from the first stage (see <FIG>) approaches the life of the second stage resonator and the coupling efficiency improves.

In <FIG>, a horizontal axis is the time for the Q factor of the first stage resonator to become half, and a vertical axis is a peak value of the output pulse of the second stage resonator. The Q factor is changed with a convex upward quadratic function in the example herein, but may be changed with another appropriate function. It can be understood that the coupling efficiency can be optimum by appropriately selecting the time to lower the Q factor.

<FIG> is a schematic diagram of a pulse generator <NUM> according to a third embodiment. In the third embodiment, a resonator length (a round-trip length L1) in a first stage is set long by using an optical fiber, and a pulse width of a first stage output pulse is widened. A longer resonator length can be set in a space-saving manner by using the optical fiber. The pulse width of the output pulse of the first stage resonator is widened to increase energy of light incident on a second stage resonator, and pulse light having high peak power is extracted from the second stage resonator.

The pulse generator <NUM> includes a Q switch resonator 10E in the first stage, and a Q switch resonator <NUM> in the second stage to which a pulse output from the Q switch resonator 10E is input. The Q switch resonator 10E in the first stage includes a mirror <NUM>, a mirror <NUM>, an optical fiber <NUM>, and a Q switch <NUM>. The optical fiber <NUM> is used for at least a portion of an optical path between the mirror <NUM> and the mirror <NUM>, and is configured to resonate light.

<FIG> is a configuration example of the first stage resonator used in the pulse generator <NUM> in <FIG>. A Q switch resonator 10F includes an optical fiber <NUM>, the mirror <NUM>, and the Q switch <NUM>. A reflective surface <NUM> is formed on one end side of the optical fiber <NUM> by applying a reflective coating, for example. The reflective surface <NUM> also serves as an incident surface of CW light. Another end <NUM> of the optical fiber <NUM> serves as an emission end to the mirror <NUM>.

The reflective surface <NUM> of the optical fiber <NUM> and the mirror <NUM> constitute an optical resonator. The mirror <NUM> may be provided with an actuator <NUM> for adjusting a round-trip length of the resonator to an integral multiple of a wavelength of a CW laser <NUM>. Light incident on the optical fiber <NUM> is repeatedly reflected and is amplified between the mirror <NUM> and the reflective surface <NUM>. While energy is stored in the resonator, the Q factor of the resonator is maintained high. When the energy in the resonator approaches a steady value, the Q factor is switched to a low side and pulse light is output from the resonator. At this time, it is preferable to control the switching time of the Q factor to improve the coupling efficiency to the second stage resonator.

<FIG> is a configuration example of a pulse generator <NUM> having the configuration in <FIG>. The configuration of the pulse generator <NUM> is substantially the same as that of the pulse generator <NUM> in <FIG>, but is different in that a Q switch resonator <NUM> in the first stage uses an optical fiber <NUM>.

The Q switch resonator <NUM> constitutes a triangular ring-type optical resonator with mirrors <NUM>, <NUM>, and <NUM> and the optical fiber <NUM>, and a Q switch <NUM> is provided in the optical resonator. In this configuration example, no reflective coating is applied to the end surfaces of the optical fiber <NUM>, and an adjustment is made such that incident CW light or resonating reflected light is condensed on an end surface of the optical fiber <NUM> using a curvature of the mirror <NUM>. Any of the mirrors <NUM>, <NUM>, and <NUM> may be provided with the actuator <NUM> for adjusting a round-trip length of the resonator.

An output pulse of the Q switch resonator <NUM> is input to a Q switch resonator 20A in the second stage by an optical system <NUM> for a lateral mode adjustment. When energy stored in the Q switch resonator 20A approaches a steady value, the Q switch <NUM> switches the Q factor of the resonator to a low side and outputs a shorter optical pulse having higher peak power than the first stage output.

<FIG> and <FIG> are graphs illustrating resonator characteristics of the third embodiment. <FIG> illustrate characteristics of the first stage resonator, and <FIG> illustrate characteristics of the second stage resonator. Parameters of a simulation are set as follows:.

A resonator length of the first stage resonator and the second stage resonator is controlled to be an integral multiple of a wavelength of the CW laser <NUM>.

<FIG> illustrates a transmittance AQ(<NUM>) of the Q switch <NUM> in the first stage, <FIG> illustrates a Q factor of the first stage resonator, <FIG> illustrates power in the first stage resonator, and <FIG> illustrates a pulse output of the first stage resonator. An initial value of AQ(<NUM>) of the Q switch <NUM> in the first stage is set to the maximum value (<NUM>).

The power accumulated in the first stage resonator gradually increases from a start of incidence. When the power in the first stage resonator approaches a steady value, the AQ(<NUM>) value is lowered to <NUM> for a time of about <NUM> microsecond to extract pulse light in the first stage. The Q factor of the first stage resonator and the power in the resonator decrease slower than those in the first embodiment, and a pulse output in the first stage is acquired.

In <FIG>, an initial value of AQ(<NUM>) of the Q switch <NUM> in the second stage is the maximum value (<NUM>), and the Q factor is set high from the beginning. In <FIG>, the power in the second stage resonator rapidly rises immediately after an input of the first stage pulse. When the power in the second stage resonator reaches a peak, AQ(<NUM>) is lowered to <NUM> for a time of <NUM> nanosecond to extract a pulse of interest. The power in the second stage resonator decreases at once, and the short pulse in <FIG> is obtained.

<FIG> and <FIG> are enlarged views near <NUM> microseconds in <FIG> and <FIG>. Corresponding to <FIG> and <FIG>, <FIG> illustrates a transmittance AQ(<NUM>) in the first stage, <FIG> illustrates a Q factor of the first stage resonator, <FIG> illustrates power in the first stage resonator, and <FIG> illustrates a first stage pulse output.

A pulse width when the AQ(<NUM>) value is immediately set to <NUM> is <NUM> nanoseconds, but in this example, the AQ(<NUM>) value in the first stage is slowly lowered to <NUM> over a period of time of approximately <NUM> microsecond, and thus a pulse width of the first stage pulse output is about <NUM> microsecond, which is long.

In <FIG> and <FIG>, the transmittance AQ(<NUM>) in the second stage is changed to <NUM> for a time of <NUM> nanosecond at a point in time when the power in the second stage resonator reaches a peak. The power in the second stage resonator decreases at once, and the pulse in <FIG> is acquired. This pulse is output at a maximum output of <NUM> W immediately after the Q switch <NUM> in the second stage is switched, and becomes a half value after about <NUM> nanoseconds from the output.

Thus, by using the optical fiber in the Q switch resonator 10D in the first stage and inputting the output pulse of the Q switch resonator 10D to the Q switch resonator 20A in the second stage, a short pulse having high peak power can be obtained. In the third embodiment, although there is a slight light loss to the fiber or from the fiber due to an input/output, a round-trip length of the resonator can be set long, and a degree of freedom in design is high, such as allowing generation of a pulse having high energy per pulse in a relatively space-saving manner.

The present invention is described above based on specific examples, but the present invention is not limited to the configuration examples described above. For example, an optical resonator constituting a Q switch resonator is not limited to a resonator using two mirrors or three mirrors, but may be a bow tie-type resonator using four mirrors. Also in this case, a part of the mirrors constituting the optical resonator may be used as a Q switch. Further, an optical fiber may be applied to at least a portion of the optical resonator.

An adjustment of single lateral mode and/or an adjustment of single longitudinal mode between a CW light source and an optical resonator is not limited to the illustrated examples, and an appropriate control mechanism may be adopted depending on an arrangement configuration of the optical resonator.

Claim 1:
A Q switch resonator (<NUM>), comprising:
an optical resonator formed of at least two mirrors (<NUM>, <NUM>), and configured to accumulate power of a continuous wave or an intermittent continuous wave incident from an outside; and
a switching element (<NUM>) provided in the optical resonator, wherein,
when the power accumulated in the optical resonator increases to a predetermined level, the switching element (<NUM>) outputs an optical pulse by lowering a Q factor from a first level to a second level lower than the first level;
characterized in that
the optical resonator does not contain a gain medium inside the optical resonator.