Differential absorption laser radar gas detection apparatus having tunable wavelength single mode semiconductor laser source

• US 5,015,099 A
• Filed: 03/12/1990
• Issued: 05/14/1991
• Est. Priority Date: 03/23/1989
• Status: Expired due to Fees

• Current Assignee: Anritsu Corporation
• Original Assignee: Anritsu Corporation
• Inventors: Nagai, Haruo, Aizawa, Masafumi, Uehara, Kiyoji
• Primary Examiner(s): Davie, James W.
• Application Number:
  US07/492,784
• Time in Patent Office: 428 Days
• Field of Search: 372/20, 372/31, 372/32, 356/437, 356/438, 250/205, 250/339, 250/343, 250/575
• US Class Current: 356/437
• CPC Class Codes: G01N 2021/394 DIAL method
• CPC Class Codes: G01N 21/39 using tunable lasers

Abstract

A light source has a tunable wavelength single mode semiconductor laser. The laser outputs a single mode laser beam having a wavelength tuned in accordance with a drive current and emits, for an object to be detected, at least a first laser beam having a first wavelength which is absorbed by a gas to be detected and a second laser beam having a second wavelength which is not absorbed by the gas to be detected. A controller switches the drive current having a predetermined value corresponding to the first or second wavelength at a predetermined period and supplies the drive current to the laser and controls the drive current having the predetermined value corresponding to the first or second wavelengths so that the first or second laser beams are alternately output from the laser with substantially the same power while the first and second wavelengths are maintained. A light-receiving unit receives first and second response light components generated when the first and second laser beams emitted alternately from the laser are incident on the object to be detected and outputs electrical signals corresponding to received light amounts of the first and second response light components. A signal processor receives the electrical signals from the light-receiving unit and processes the presence/absence of gas detection in accordance with a difference between the received light amounts of the first and second response light components.

Claims

1. A gas detection apparatus, comprising:

light source means including a tunable wavelength single mode semiconductor laser, said tunable wavelength single mode semiconductor laser including means for outputting a single mode laser beam having a wavelength tuned in accordance with a drive current to a first optical path and a second optical path, and means for emitting, for an object to be detected, at least a first laser beam having a first wavelength which is absorbed by a gas to be detected or a second laser beam having a second wavelength which is not absorbed by the gas to be detected;

first control means for switching the drive current having a predetermined value corresponding to the first or the second wavelength at a predetermined period, and for supplying the drive current to said tunable wavelength single mode semiconductor laser, said first control means including means for controlling the drive current having the predetermined value corresponding to the first or the second wavelength so that the first and the second laser beams are alternately output from said tunable wavelength single mode semiconductor laser with substantially the same power while the first and the second wavelengths are maintained;

first light-receiving means for receiving first and second response light components generated when the first and the second laser beams emitted alternately from said tunable wavelength single mode semiconductor laser are incident on the object to be detected through the first optical path, and for outputting electrical signals corresponding to received light amounts of the first and the second response light components;

signal processing means for receiving the electrical signals from said first light-receiving means, and for processing a presence/absence condition of gas detection in accordance with a difference between the received light amounts of the first and the second response light components;

second light-receiving means for receiving the first and the second laser beams emitted from said tunable wavelength single mode semiconductor laser through the second optical path, and for outputting detection signals corresponding to the wavelengths and powers of the received first and second laser beams, said second light-receiving means including a wavelength divider for selecting said first laser beam, a first light-receiving element for receiving a beam output from the wavelength divider, and a second light-receiving element for receiving the first and the second laser beams non-selectively; and

second control means for feeding back a correction signal for correcting the drive currents having the predetermined values to said first control means in accordance with the detection signal from said second light-receiving means.

2. An apparatus according to claim 1, wherein the response light components comprise light reflected by a portion around the object to be detected.

3. An apparatus according to claim 1, wherein the response light components comprise light transmitted through the object to be detected.

4. An apparatus according to claim 1, wherein said tunable wavelength single mode semiconductor laser comprises a 3-electrode Distributed Bragg Reflector (DFB) laser having first, second and third electrodes, and said first control means includes means for flowing a sum I₂ of currents to be flowed to said first and said third electrodes, and for flowing a current I₁ to said second electrode, thereby controlling

I₁ /(I₁ +I₂) to be a desired value.

5. An apparatus according to claim 4, wherein, when said second light-receiving element receives the first laser beam, said second control means controls a value (n) of I₁ /(I₁ +I₂) so that the output from the first light-receiving element takes a maximum value, holds the value (n), and controls (I₁ +I₂) so that the output from the first light-receiving element assumes a most desirable value.

6. An apparatus according to claim 4, wherein, when said second light-receiving element receives the first laser beam, said second control means controls a value (n) of I₁ /(I₁ +I₂) so that the output from the first light-receiving element takes a minimum value, holds the value (n), and controls (I₁ +I₂) so that the output from the first light-receiving element assumes a most desirable value.

7. An apparatus according to claim 5, wherein, when said first light-receiving element receives the first laser beam, said second control means scans the value (n) at predetermined intervals, and calculates the most desirable value (n) on the basis of values found by the scan.

8. An apparatus according to claim 6, wherein, when said first light-receiving element receives the first laser beam, said second control means scans the value (n) at predetermined intervals, and calculates the most desirable value (n) on the basis of values found by the scan.

9. An apparatus according to claim 3, wherein said apparatus further comprises:

a first optical fiber for guiding the first and the second laser beams to locations for measurement;

a first lens for emitting the guided laser beams to a space;

a second lens for collecting the laser beams emitted to the space and passed through the gas to be detected; and

a second optical fiber for guiding the collected laser beams to said first light-receiving means.

10. A method for detecting a gas, said method comprising the steps of:

providing light source means including a tunable wavelength single mode semiconductor laser, said tunable wavelength single mode semiconductor laser outputting a single mode laser beam having a wavelength tuned in accordance with a drive current to a first optical path and a second optical path, the semiconductor laser emitting, for an objected to be detected, at least a first laser beam having a first wavelength which is absorbed by a gas to be detected or a second laser beam having a second wavelength which is not absorbed by the gas to be detected;

switching the drive current having a predetermined value corresponding to the first or the second wavelength at a predetermined period and supplying the drive current to said tunable wavelength single mode semiconductor laser by first control means, and controlling the drive current having the predetermined value corresponding to the first or the second wavelength so that the first and second laser beams are alternately output from said tunable wavelength single mode semiconductor laser with substantially the same power while the first and the second wavelengths are maintained;

receiving the first and the second laser beams emitted from the tunable wavelength single mode semiconductor laser through the second optical path by first light-receiving means, and outputting detection signals corresponding to the wavelengths and powers of the received first and second laser beams;

feeding back a correction signal for correcting the drive currents having the predetermined values to the first control means by second control means in accordance with the detection signal from the first light-receiving means;

receiving first and second response light components generated when the first and the second laser beams emitted alternately from the tunable wavelength single mode semiconductor laser are incident on the object to be detected through the first optical path by second light-receiving means, and outputting electrical signals corresponding to received light amounts of the first and the second response light components; and

receiving the electrical signals from the second light-receiving means by signal processing means, and processing a presence/absence condition of gas detection in accordance with a difference between the received light amounts of the first and the second response light components.

11. A gas detection apparatus, comprising:

light source means including a tunable wavelength single mode semiconductor laser, said tunable wavelength single mode semiconductor laser including means for outputting a single mode laser beam having a wavelength tuned in accordance with a drive current and emitting, for an object to be detected, at least a first laser beam having a first wavelength which is absorbed by a gas to be detected or a second laser beam having a second wavelength which is not absorbed by the gas to be detected;

control means for switching the drive current having a predetermined value corresponding to the first or the second wavelength at a predetermined period, and for supplying the drive current to said tunable wavelength single mode semiconductor laser, said control means including means for controlling the drive current having the predetermined value corresponding to the first or the second wavelength so that the first and the second laser beams are alternately output from said tunable wavelength single mode semiconductor laser with substantially the same power while the first and the second wavelengths are maintained;

light-receiving means for receiving first and second response light components generated when the first and the second laser beams emitted alternately from said tunable wavelength single mode semiconductor laser are incident on the object to be detected, and for outputting electrical signals corresponding to received light amounts of the first and the second response light components wherein the first and the second response light components are transmitted through the object to be detected;

signal processing means for receiving the electrical signals from said light-receiving means, and for processing a presence/absence condition of gas detection in accordance with a difference between the received light amounts of the first and the second response light components;

transmitting means for storing at least said light source means; and

receiving means for storing at least said light-receiving means;

wherein said transmitting means and said receiving means include means for performing data communication using the second wavelength between one another.