JPH11326199A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a sub-mode suppression ratio and to detect gas accurately and inexpensively by a gain-coupling distribution feedback(GC-DFB) laser as the light source of a laser beam. SOLUTION: As a device for light source of a laser beam, a semiconductor laser 6 consisting of a GC-DFB laser that causes less sub-mode suppression ratio due to return light to decrease and cannot become multiple mode easily is provided on a mounting stand 5, thus eliminating the need for executing, especially measures against return light (oblique machining and layout), at a condensing lens 8, a reference gas cell 10, and a light receiver 11 at the side of a reference gas, and eliminating the need for an expensive isolator for preventing the passage of reflection light. Therefore, even if return light due to reflection occurs, the reduction of the sub-mode suppression ratio is small, gas can be detected accurately, and the costs of a device can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detecting device, and more particularly to a gas detecting device for performing spectroscopic analysis of a gas to be detected using a semiconductor laser.

[0002]

2. Description of the Related Art Conventionally, chemical gas detectors such as gas chromatographs have been used for measuring gas concentrations in the atmosphere and detecting gas leaks from city gas or chemical plants.

[0003] However, it is difficult to perform simple and quick measurement with such a gas detection device that performs such a chemical measurement, and in consideration of this point, a spectroscopic gas detection device using a laser is considered. Is being developed.

[0004] This spectroscopic gas detector utilizes the fact that a gas absorbs light of a specific wavelength according to its type. Generally, gas often absorbs electromagnetic waves in the infrared region. For example, methane absorbs light having a wavelength of 3.3 nm. Therefore, if a laser beam of a specific wavelength is emitted to the space to be measured and its attenuation state is measured, it is possible to detect gas leakage extremely simply and quickly.

[0005] By the way, the laser light source used in such an apparatus is required to be small as a premise.
Although a gas laser or the like can obtain a single-mode wavelength light with high output, it does not satisfy the requirement of the above-mentioned compact size, and it is considered that a semiconductor laser can be a practically usable laser light source. The fact that the emitted laser light is in a single mode is an extremely important factor in achieving high-precision detection of the gas to be detected due to the configuration of the device that absorbs light of a certain wavelength and performs measurement. It is.

However, semiconductor lasers are inherently multi-mode oscillations, and single-mode oscillations are usually realized by various measures. However, when there is return light to the laser element, the sub-mode suppression ratio decreases. It has been conventionally considered that multi-mode oscillation occurs.

That is, when there is return light due to reflection, the emission state of the laser light itself fluctuates, and the characteristics of the measurement device light source fluctuate. In order to prevent the light source characteristics from fluctuating due to the decrease in the submode suppression ratio, various devices have been conventionally employed so that the reflected light does not return to the laser element.

FIG. 9 is a diagram showing a configuration example of a semiconductor laser module in a conventional gas detection device. In this semiconductor laser module, laser light from a semiconductor laser 71 passes through an aspheric lens 72, an isolator 73, and a protective glass 74 and is incident on a gas to be measured. Laser light is also emitted from the semiconductor laser 71 in the opposite direction, and the aspheric lens 76 and the isolator 7
7. The light passes through the reference gas cell 78 and enters the photodetector 79.

In the gas detection, the laser wavelength is controlled by temperature control by the Peltier element 80 based on the light reception measurement through the reference gas cell 78 in which the gas to be detected is sealed.
The laser 71 whose wavelength is controlled in this way and whose output is stabilized
Is attenuated by passing through the gas to be measured. Then, the transmitted light is measured by a measurement system (not shown), and the concentration of the gas to be detected in the gas to be measured is output.

Here, consideration is made so that the reflected return light from the two optical paths to the semiconductor laser 71 is minimized. First, aspheric lenses 72 and 76 are arranged obliquely with respect to the optical axis, and isolators 73 and 77 are provided. The isolator rotates the plane of polarization of light passing through the crystal, and blocks the passage of reflected light. Also, the protective glass 74 on the measured gas side is provided obliquely to the optical axis in order to reduce reflected light.

On the other hand, also on the side of the reference gas cell 78, the cell end faces 78a and 78b of the reference gas cell 78 are inclined with respect to the optical axis so that light is hardly reflected. Further, the photodetector 79 itself has an oblique detection surface. Thus, the conventional semiconductor laser module for a gas detection device is configured so that various return lights are not generated as much as possible.

[0012]

As described above, in the conventional gas detector using the semiconductor laser, the protective glass 74 and the cell end faces 78a and 78b are processed obliquely to prevent the return of the reflected light, and the isolator 73 is used. , 77 and the like must be used. If such a configuration is not provided, the sub-mode suppression ratio decreases, the laser oscillation becomes multimode, and the measurement accuracy decreases. In some cases, measurement becomes impossible.

On the other hand, if oblique machining or an isolator is used to prevent this inconvenience as in the above-described apparatus, the manufacturing cost of the apparatus increases, and it becomes difficult to provide an inexpensive gas detection apparatus.

The present invention has been made in view of such circumstances, and even in a case where a semiconductor laser is used as a light source and return light due to reflection can occur, the reduction in the submode suppression ratio is small and high accuracy and high accuracy are achieved. It is an object of the present invention to provide a gas detection device that enables low-cost gas detection.

[0015]

The gist of the present invention is that a gain-coupled distributed feedback (GC) is used as a laser light source for a gas detection device.
-DFB) or composite coupled distributed feedback type (CC-DF
B) The use of a laser. The present invention has been made based on findings from the inventors' experimental results. In the above description, it has been described that the semiconductor laser generally becomes multi-mode when there is return light. However, the inventors have found that a certain type of semiconductor laser does not easily become multi-mode even when there is return light.

The present inventors have conducted detailed experiments on this point, and found that the sub-mode suppression ratio of the GC-DFB laser and the CC-DFB laser hardly changes even if the return light increases to about 10%. (See FIGS. 5 and 6). The details of this experiment will be described later.

[0017] The invention corresponding to claim 1 is based on the above knowledge, and when laser light of a predetermined wavelength is incident on a gas to be measured, the laser light is attenuated by the gas to be detected contained in the gas to be measured. This is a gas detection device that uses a gain-coupled distributed feedback laser as a light source of a laser beam in a gas detection device that performs gas detection using a laser beam.

Since the present invention has such a configuration, even if there is a return light due to reflection or the like to the laser light oscillation section, the laser light of a constant wavelength is stably received without multimode operation. It can continue to be incident on the measuring gas. Therefore, stable and highly accurate gas detection measurement can be performed, and expensive parts such as an isolator and anti-reflection processing are not required in the laser oscillation unit. Can be provided.

A second aspect of the present invention is a gas detecting apparatus according to the first aspect of the present invention, which uses a composite-coupled distributed feedback laser instead of a gain-coupled distributed feedback laser as a light source.

According to the present invention, the same effects as those of the first aspect can be obtained. Further, in connection with the inventions corresponding to the first and second aspects, the present inventors have proposed that a conventional index-coupled distributed feedback (IC-DFB) laser increases an injection current to increase a change in wavelength. If the probability of multi-mode and mode hop is high,
-It has been found that DFB and CC-DFB have a high probability of maintaining a single mode. Therefore, GC-DFB and CC-D
By using FB as a light source, a gas detection device having a wide wavelength tuning range can be provided.

[0021]

Embodiments of the present invention will be described below. FIG. 1 is a diagram showing a configuration example of a semiconductor laser module in a gas detection device according to a first embodiment of the present invention.

In this semiconductor laser module,
A base 3 is provided on a substrate 2 inside the cylinder of the cylindrical case 1, and a mount 5 is provided on a Peltier element (temperature control element) 4 mounted on the surface of the base 3.

A semiconductor laser 6 composed of a GC-DFB or CC-DFB laser is provided on the mounting base 5 and is arranged to emit laser light along the central axis of the cylindrical case 1. The semiconductor laser 6 is used to measure the concentration of the gas to be detected, and the temperature is controlled by the Peltier element 4 to control the laser wavelength to match the gas to be detected. The laser light is emitted to both the measured gas side and the reference gas side.

Condensing lenses 7 and 8 are provided on the mounting base 5 for condensing the laser beams emitted to the measured gas side and the reference gas side into parallel beams, respectively. Are located in

Light from the semiconductor laser 6 to the gas to be measured is output to the outside via the condenser lens 7 and the protective glass 9 for protecting the semiconductor laser module, and is incident on the gas to be measured. Here, the condenser lens 7 and the protective glass 9 are not particularly obliquely processed with respect to the optical axis, and have appropriate shapes in consideration of cost and the like. Further, no isolator is provided.

On the other hand, light from the semiconductor laser 6 to the reference gas side is converted into a parallel beam by the condenser lens 8, and further received by the light receiver 11 via the reference gas cell 10. Here, the reference gas cell 10 is a cell in which a gas to be detected is sealed as a reference gas, and is for adjusting the wavelength of the laser beam to the absorption wavelength of the gas to be detected through a wavelength stabilization control circuit described later. The light receiver 11 converts the received laser light into an electric signal (current).

Here, as in the case of the gas to be measured,
No isolator is provided, and no countermeasures against return light (oblique processing, arrangement, etc.) are provided in the condenser lens 8, reference gas cell 10, and light receiver 11.

In the semiconductor laser module shown in FIG. 1, it is not necessary to take measures against return light due to reflection.
This is because in the C-DFB or CC-DFB laser, the reduction of the sub-mode suppression ratio due to the return light is small, and it is difficult for the mode to be multi-mode. In this regard, the inventors' experiments will be described.

FIG. 2 is a diagram showing the configuration of the main part of each type of DFB laser that was the subject of the experiment. The figure (a) shows GC-
FIG. 1B shows the configuration of a CC-DFB laser, and FIG. 2C shows the configuration of a refractive index-coupled distributed feedback (IC-DFB) laser.

Each DFB laser has an n-InP cladding layer 51 and an n-InGaA on an InP substrate (not shown).
s P SCH layer (optical confinement layer) 52, active layer (MQ
W) 53, p-In GaAs SCH layer 54, p-I
The nP clad layer 55 is formed by sequentially laminating.

In the case of the GC-DFB laser, SC
N-InP at regular intervals between the H layer 54 and the cladding layer 55
Is provided to realize gain coupling. Similarly, for a CC-DFB laser, S
N-InGaAsP between the CH layer 54 and the cladding layer 55
Is provided.

A secondary grating is used for the GC-DFB and CC-DFB lasers. In the case of a secondary grating, light of the wavelength to be extracted is generated not only in a direction parallel to the grating but also in a direction orthogonal thereto, so that the diffraction efficiency is theoretically reduced and the maximum light output is that of the primary grating. Should be lower than. Nevertheless, according to the experiments performed by the inventors, it is possible to obtain a secondary grating having a maximum light output equivalent to that of the primary grating.

The first reason is considered to be due to the ease of manufacturing. That is, since the secondary grating is easy to manufacture, an accurate grating is formed, and top data is obtained at a position closer to the maximum theoretical value.
For this reason, as a result, top data equivalent to the primary grating which is difficult to manufacture accurately can be obtained.

The second reason is that the width of the current block layer and the OC
It is considered to be in an aspect ratio with the layer thickness of L (Optical Confinement Layer: optical confinement layer). Since the OCL is a light confinement layer, there is an optimum thickness corresponding to the manufacturing conditions in order to appropriately confine the light emitted from the active layer.
In the case of the laser of the present embodiment, it is about 90 nm. However, even if the OCL is 90 nm, a sufficient current blocking cannot be performed if the width of the current blocking layer is at most about 120 nm as in a primary grating. In this case, after the current passes through the portion without the block layer, the current wraps around to reach the active layer, so that a sufficient current difference is not given to the active layer and, consequently, a sufficient gain difference cannot be obtained. That is, the gain coupling becomes insufficient.

FIG. 3 is a diagram showing the spread of current after passing through the current blocking layer in the primary and secondary gratings. FIG. 3B shows a state in which the above-described current wraparound occurs and a sufficient gain difference cannot be obtained.
In the case of the primary grating, the characteristics that should be theoretically obtained cannot be obtained due to the current spread, and the top data is lower than the original data.

On the other hand, in the case of the secondary grating shown in FIG. 3A, the width of the current blocking layer 57 with respect to the thickness of the OCL 56 is sufficiently large as 240 nm, and the current wraparound is small. Therefore, in this case, sufficient gain coupling is obtained, and good characteristics are obtained.

As described above, the gain-coupling type current blocking layer 5
In addition to realizing the performance of the OCL, the size (ratio) of the current block layer width to the OCL thickness needs to be sufficiently large. On the other hand, OCL
The layer thickness often has an optimum value depending on the laser material, type, and manufacturing method. Therefore, in the present embodiment, the ratio is made appropriate by dare to employ a secondary grating. Therefore, in order to obtain this optimum ratio, it is conceivable to use a third-order grating or a fourth-order grating.

On the other hand, in the case of an IC-DFB laser, SC
The shape of both layers is controlled so that a diffraction grating 58 is formed between the H layer 54 and the cladding layer 55. Note that CC-
Also in the case of the DFB laser, a diffraction grating 59 is formed between the SCH layer 54 and the cladding layer 55.

FIG. 4 is a diagram showing a configuration of an experimental apparatus for evaluating the anti-return light characteristic of the DFB laser. This experimental device
The emitted light from the DFB laser 61 as shown in FIG.
The light is distributed by the B coupler 62 and enters the variable optical attenuator 63 and the optical spectrum analyzer 64.

The laser light incident on the variable optical attenuator 63 is attenuated according to the set amount, is reflected by the mirror 65, and passes through the variable optical attenuator 63 again. The return light whose reflection intensity has been adjusted in this way is distributed by the 3 dB coupler 62 and D
The light enters the FB laser 61 and the optical power meter 66.

The return light incident on the DFB laser 61 is
This is a reflected return light that affects the laser oscillation state and is a problem in this specification. The magnitude of the reflected return light is measured by the optical power meter 66.

On the other hand, after being affected by the return light, D
The laser light emitted from the FB laser 61 enters the optical spectrum analyzer 64 via the 3 dB coupler 62. The optical spectrum analyzer measures the light intensity for each wavelength.

FIG. 5 is a diagram showing the dependence of the sub-mode suppression ratio on the amount of return light. The submode suppression ratio indicates the ratio between the main mode and the submode. The larger this value is, the smaller the submode wavelength light intensity with respect to the main mode wavelength light intensity is, and the single mode oscillation is performed. Become.

In the experimental apparatus shown in FIG.
Is adjusted, the spectrum for each return light ratio is measured. As a result, FIG. 5 shows the sub-mode suppression ratio with respect to the return light ratio for each DFB laser.

FIG. 6 shows each D when the return light is present at 10%.
FIG. 3 is a diagram illustrating an optical spectrum of an FB laser. 5 and 6 show that the GC-DFB laser and the CC-D
In the FB laser, even if the return light ratio increases to about 10%, the sub-mode suppression ratio hardly decreases, indicating that single-mode oscillation is maintained.

On the other hand, in the IC-DFB laser, the return light ratio is about 1%, and the submode suppression ratio is remarkably reduced (FIG. 5).

As described above, the semiconductor laser (GC-DFB laser or CC-DFB laser) used in the present embodiment
It was confirmed that even if there was a return light due to reflection, the oscillation characteristics did not fluctuate substantially, and even if the semiconductor laser module was configured as shown in FIG. 1, no reduction in measurement accuracy was caused. Next, a configuration of a semiconductor laser oscillation unit and a light receiving measurement system using the semiconductor laser module will be described.

FIG. 7 is a diagram showing a configuration example of a semiconductor laser oscillation section in the gas detection device of the present embodiment. This semiconductor laser oscillation section is configured by adding a gas concentration measurement laser light stabilizing circuit to the semiconductor laser module 20 described in FIG. The Peltier element 4 shown in FIG. 1 is composed of an upper Peltier element 4a and a lower Peltier element 4b.

The gas concentration measuring laser light stabilizing circuit comprises a current-voltage converter 21, a fundamental wave signal amplifier 22, a signal differential detector 23, a wavelength stabilizing control circuit 24, a temperature stabilizing PID circuit 25, And a current stabilizing circuit 26.

Here, first, the current-voltage converter 21 converts the light reception current of the light receiver 11 into a voltage. The fundamental signal amplifier 22 amplifies the voltage converted by the current-voltage converter 21. The signal differential detector 23 differentiates the voltage waveform amplified by the fundamental signal amplifier 22 to generate a deviation signal from the absorption line wavelength of the gas to be detected.

The wavelength stabilization control circuit 24 performs control for stabilizing the emission wavelength of the semiconductor laser to the wavelength of the gas absorption line to be detected. That is, the shift signal from the signal differential detector 23 is converted into a semiconductor laser temperature and output to the temperature stabilizing PID circuit 25, and a control signal is output to the current stabilizing circuit 26 based on the shift signal.

The temperature stabilizing PID circuit 25 controls each Peltier element 4 (4a, 4b). That is, P is set so that the semiconductor laser 6 oscillates at a desired wavelength in accordance with the temperature signal from the wavelength stabilization control circuit 24.
ID control is performed to stably maintain the temperature of the semiconductor laser at a constant temperature.

The current stabilizing circuit 26 stabilizes the driving current of the semiconductor laser 6 in accordance with the control signal from the wavelength stabilizing control circuit 24. Next, the configuration of a light receiving measurement system that receives laser light from the semiconductor laser 6 and performs gas detection will be described.

FIG. 8 is a diagram showing a configuration example of a light receiving measurement system in the gas detection device of the present embodiment. This light receiving measurement system includes a light receiver 31 having a light receiving element 31a and a gas detector 32.

The light receiver 31 receives the laser light from the semiconductor laser module 20 that has passed through the gas to be measured 30 by the light receiving element 31a, and converts the laser light into a current. The gas detector 32 converts the received light current obtained by the light receiver 31 into a voltage signal by the current / voltage converter 33, and further uses the conversion result to output a fundamental signal detector 34 and a second harmonic signal detector 3.
The detection target gas is detected by 5 and the fundamental wave second-wave division unit 36, and a final gas detection signal is output.

The fundamental wave signal detector 34 detects the fundamental wave voltage of the converted light receiving voltage signal. The second harmonic signal detector 35 detects a second harmonic voltage of the received light voltage signal. The fundamental wave doubler divider 36 outputs a signal obtained by dividing the second harmonic by the fundamental wave as a detection signal of the concentration of the gas to be measured. It is to be noted that the signal operation (division) is performed because the measurement light has a certain level of fluctuation due to a reason other than the passage through the gas 30 to be measured. To do it.

Next, the operation of the gas detection device according to the present embodiment configured as described above will be described. First,
1 and 7, a laser beam emitted backward from a semiconductor laser 6 is condensed by a condenser lens 8, guided to a reference gas cell 10 in which a gas to be detected is sealed, and
1 and a fundamental wave is generated. Current-voltage converter 21
Is amplified by the fundamental wave signal amplifier 22, the level is detected by the signal differentiation detector 23, and a deviation signal from the absorption line wavelength of the gas to be detected is generated.

This signal is converted into a semiconductor laser temperature by a wavelength stabilization control circuit 24, and a temperature stabilization PID circuit 25
The Peltier element 4 is controlled by the PID control of the temperature stabilizing PID circuit 25 so that the above-mentioned deviation is eliminated.
The temperature of (4a, 4b) is adjusted. The temperature of the entire semiconductor laser mounting portion is stabilized by the lower Peltier element 4b, and fine adjustment of laser oscillation is performed by the upper Peltier element 4a.

When the above-mentioned signal shift is eliminated, the wavelength of the semiconductor laser 6 is adjusted to the wavelength absorbed by the gas in the reference gas cell 10. Therefore, at this time, the current to the semiconductor laser 6 is also stabilized by the control of the wavelength stabilization control circuit 24, and the wavelength output of the laser light is also stabilized, so that the gas concentration can be stably measured.

During the laser stabilization process, the laser light is reflected by the condenser lenses 7, 8 and the protective glass 9 or the reference gas cell 10, and is incident on the semiconductor laser 6 as return light. However, as shown in FIGS. 5 and 6, the semiconductor laser 6 of the present embodiment does not become multimode even when there is return light, and stable laser oscillation is continued.

Thus, the laser light emitted from the front of the semiconductor laser passes through the gas to be measured 30 and is received by the light receiver 31 shown in FIG. The light receiving signal is converted into a voltage signal by the current / voltage converting circuit 33, and the fundamental wave level of the light receiving voltage signal is detected by the fundamental wave signal detecting circuit. On the other hand, the second harmonic level is detected from the converted voltage signal.

This second harmonic wave level is divided by the fundamental wave level divider 36 in the fundamental wave second harmonic wave divider 36, and the level fluctuation is removed and output as a gas concentration detection signal. As described above, the gas detection device according to the embodiment of the present invention includes:
GC-DFB or CC as laser element of semiconductor laser
-Since the DFB laser is used, it is possible to prevent a decrease in the submode suppression ratio due to the reflected return light, and to prevent a multimode operation. Therefore, highly sensitive gas detection can be performed even when the semiconductor laser module is configured to generate return light due to reflection, and oblique processing of the light passage portion and installation of an isolator are not required for performing the highly sensitive gas detection. Yes, it can be a low-cost device.

The present invention is not limited to the above embodiments, but can be variously modified without departing from the gist thereof. For example, in the embodiment, GC
The case where a semiconductor laser module using a -DFB or CC-DFB laser is used for a gas detection device has been described, but the present invention can be applied to a device using similar laser light absorption. For example, it can be applied to a spectroscopic analyzer or an infrared analyzer. In addition, the present invention can be applied to a fiber temperature sensor that measures temperature using Brillouin scattered light from an optical fiber, a stress sensor using Raman scattering, and the like.

[0064]

As described above in detail, according to the present invention, since the GC-DFB or CC-DFB laser is used as the laser element for the light source, the semiconductor laser is used as the light source and return light due to reflection may occur. Even in such a case, it is possible to provide a gas detection device that can perform high-accuracy and low-cost gas detection with a small decrease in the submode suppression ratio.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration example of a semiconductor laser module in a gas detection device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a main part of each type of DFB laser as an experiment target.

FIG. 3 is a diagram showing a state of current spreading after passing through a current blocking layer in the primary and secondary gratings.

FIG. 4 is a diagram showing the configuration of an experimental apparatus for evaluating the return light resistance characteristics of a DFB laser.

FIG. 5 is a graph showing the dependence of the sub-mode suppression ratio on the amount of return light.

FIG. 6 is a diagram showing an optical spectrum of each DFB laser when 10% of return light exists.

FIG. 7 is a diagram showing a configuration example of a semiconductor laser oscillation unit in the gas detection device of the embodiment.

FIG. 8 is a diagram showing a configuration example of a light receiving measurement system in the gas detection device of the embodiment.

FIG. 9 is a diagram showing a configuration example of a semiconductor laser module in a conventional gas detection device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Cylindrical case 2 ... Substrate 3 ... Base 4 ... Peltier element 4a ... Upper Peltier element 4b ... Lower Peltier element 5 ... Mounting base 6 ... Semiconductor laser 7, 8 ... Condensing lens 9 ... Protective glass 10 ... Reference gas cell 11 ... Receiver 20 ... Semiconductor laser module 21 ... Current-voltage converter 22 ... Basic wave signal amplifier 23 ... Signal differential detector 24 ... Wavelength stabilization control circuit 25 ... Temperature stabilization PID circuit 26 ... Current stabilization circuit 30 ... Measurement Gas 31 photodetector 31a photodetector 32 gas detector 33 current-voltage converter 34 fundamental signal detector 35 second harmonic signal detector 36 fundamental second harmonic divider 51 n-In P cladding layer 52 n-In GaAs P SCH layer 53 active layer 54 p-In GaAs P SCH layer 55 p-In P cladding layer

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tomoyuki Kikukawa 5-107 Minami-Azabu, Minato-ku, Tokyo Anritsu Corporation (72) Inventor Takeshi Takemoto 5-10-27 Minami-Azabu, Minato-ku, Tokyo Anritsu Inside the corporation

Claims (2)

[Claims] A laser beam having a predetermined wavelength is applied to a gas to be measured (3).
0), the laser light is attenuated by the gas to be detected contained in the gas to be measured, thereby performing gas detection. Gas detection device characterized by using a type laser. 2. The gas detector according to claim 1, wherein a complex-coupled distributed feedback laser is used as the light source instead of the gain-coupled distributed feedback laser.