US20200251877A1 - Master Oscillator Power Amplifier - Google Patents
Master Oscillator Power Amplifier Download PDFInfo
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- US20200251877A1 US20200251877A1 US16/268,315 US201916268315A US2020251877A1 US 20200251877 A1 US20200251877 A1 US 20200251877A1 US 201916268315 A US201916268315 A US 201916268315A US 2020251877 A1 US2020251877 A1 US 2020251877A1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
- H01S3/2308—Amplifier arrangements, e.g. MOPA
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
- H01S3/10015—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by monitoring or controlling, e.g. attenuating, the input signal
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
- G01S17/06—Systems determining position data of a target
- G01S17/08—Systems determining position data of a target for measuring distance only
- G01S17/10—Systems determining position data of a target for measuring distance only using transmission of interrupted, pulse-modulated waves
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/481—Constructional features, e.g. arrangements of optical elements
- G01S7/4814—Constructional features, e.g. arrangements of optical elements of transmitters alone
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/483—Details of pulse systems
- G01S7/484—Transmitters
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/06—Construction or shape of active medium
- H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
- H01S3/067—Fibre lasers
- H01S3/06754—Fibre amplifiers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
- H01S3/1001—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by controlling the optical pumping
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K7/00—Modulating pulses with a continuously-variable modulating signal
- H03K7/08—Duration or width modulation ; Duty cycle modulation
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/481—Constructional features, e.g. arrangements of optical elements
- G01S7/4818—Constructional features, e.g. arrangements of optical elements using optical fibres
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/09—Processes or apparatus for excitation, e.g. pumping
- H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
- H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
- H01S3/094076—Pulsed or modulated pumping
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
- H01S3/1301—Stabilisation of laser output parameters, e.g. frequency or amplitude in optical amplifiers
- H01S3/13013—Stabilisation of laser output parameters, e.g. frequency or amplitude in optical amplifiers by controlling the optical pumping
Definitions
- the present invention relates to a master oscillator power amplifier (MOPA) and, more particularly, to a fiber-based MOPA configured to provide output pulses of constant energy regardless of changes in the input signal pulse repetition rate.
- MOPA master oscillator power amplifier
- a MOPA is known in the art as a specific type of optical amplifier that comprises at least two separate elements, a laser source (the “master oscillator”) and an optical amplifier.
- the laser source is referred to as the “seed laser”.
- the laser source is used to “seed” an optical amplifier with an input trigger signal that then generates a high power output signal pulse.
- the MOPA technology provides an efficient power scaling architecture for pulsed laser applications such as “light detection and ranging” (LIDAR).
- LIDAR is a technology that can be used to measure distances to remote targets, with a laser source used to generate optical pulses that are amplified and directed toward a target which then scatters the light.
- the separate power amplifier within the MOPA can thus be independently controlled to provide the desired amount of signal gain for a given LIDAR application.
- a relatively high power optical pulse is required so that the scattered, returned light has enough power to yield accurate distance calculations.
- the MOPA is configured to generate extremely narrow output pulses and thus requires the use of nsec-scale seed laser pulses.
- the amplifier portion typically comprises a fiber-based (rare-earth) amplifier (such as an erbium-doped fiber amplifier, EDFA) that utilizes pump light at an appropriate wavelength (e.g., 980 nm) to excite the rare-earth ions in the fiber and thereby amplify the seed laser input signal pulses to a power level sufficient for the required “high power” output pulses.
- the seed laser is controlled to exhibit a predetermined pulse repetition rate. Instead of defining the input signal pulse train in terms of repetition rate, it is also common to define the pulse train by its “pulse repetition interval” (PRI), which defines the time interval between adjacent pulses (typically measured from the rising edge of a first pulse to the rising edge of a second pulse).
- PRI pulse repetition interval
- MOPA master oscillator power amplifier
- PRI input signal pulse repetition rate
- a fiber-based MOPA is configured to utilize a pump source that operates in pulse mode (rather than CW, as in the prior art), with the arrival time of the pump pulses coordinated with the arrival time of the input seed pulses.
- pulse mode rather than CW, as in the prior art
- the use of a pulsed pump is described in detail in U.S. Pat. No. 7,100,167, entitled “System and Method for Dynamic Range Extension and Stable Low Power Operation of Optical Amplifiers Using Pump Laser Pulse Modulation”, issued on Sep. 19, 2006 to A. Gurusami et al. and herein incorporated by reference.
- the fiber-based MOPA of the present invention also controls the width of the pump pulses (as well as their arrival time), thus providing a mechanism for controlling both the amount of pump energy injected into the amplifier, as well as the overlap in time between the pump pulse and the input pulse.
- the timing of the pump pulses and their width are also changed so that a “constant gain” environment is created within the amplifying medium, providing an essentially constant energy output pulse, regulating the amount of ASE generated during different PRIs.
- One embodiment of the present invention takes the form of a MOPA including a section of doped optical fiber for providing signal gain to input light in the presence of a pump light beam operating at a defined wavelength, an input pulse source for generating seed pulses applied as a first input to the section of doped optical fiber, the input pulse source designed to adjust a pulse repetition interval (PRI) between adjacent pulses in response to a “PRI change” control signal, an a pump source for generating pump pulses applied as a second input to the section of doped optical fiber.
- PRI pulse repetition interval
- the presence of the seed and pump pulses within the section of doped optical fiber amplifies the power of the seed pulses and generates high-power optical output pulses.
- the pump pulse source itself is designed to control pump pulse parameters (including repetition rate and pulse width) to maintain an essentially constant energy in the high-power optical output pulses regardless of the seed pulse PRI.
- FIG. 1 is a simplified diagram of a prior art LIDAR system, useful for understanding an exemplary implementation of the constant pulse energy MOPA of the present invention
- FIG. 2 is a diagram illustrating a sequence of four different PRIs, utilized in sequence to control a pulsed laser in an exemplary MOPA;
- FIG. 3 illustrates an exemplary constant energy MOPA formed in accordance with the present invention
- FIG. 4 illustrates an alternative embodiment of a MOPA formed in accordance with the present invention, illustrating a look-up table used to correlate changes in PRI with pump pulse adjustments to maintain constant gain;
- FIG. 5 shows yet another embodiment of a MOPA formed in accordance with the present invention, in this case including a delay element to control the arrival times of seed pulses and pump pulses at the fiber amplifier;
- FIG. 6 illustrates an embodiment of the present invention where the inventive MOPA further comprises a feedback path between the fiber amplifier output and the pump source input.
- FIG. 1 illustrates an exemplary LIDAR system 1 used to measure the distance D between system 1 and a target TAR.
- the target may be a second vehicle traveling down a road in front of the vehicle equipped with LIDAR system 1 .
- System 1 includes a light source 2 that generates high-power light signal pulses 3 , pulses 3 being of a predetermined wavelength ⁇ sig and separated in time by a predetermined “pulse repetition interval”, or simply PRI.
- Pulses 3 pass through a beam splitter 4 and exit system 1 . As pulses 3 reach target TAR, some of the reflected/scattered light is directed back toward LIDAR system 1 as returned pulses 5 . Returned pulses 5 are thereafter re-directed by beam splitter 4 into a receiver 6 that is configured to perform a programmed analysis of the pulses to determine, for example, the distance D between LIDAR system 1 and target TAR.
- a “long” PRI i.e., a relatively long time between adjacent pulses
- a long PRI allows the system to “see” vehicles far ahead or behind and make appropriate decisions in a timely fashion (particularly required in dynamic situations where movement of targets occurs).
- the advantages of using a long PRI come at the cost of reduced spatial resolution in the x-y plane of the pulse (see FIG. 1 ).
- a “short” PRI i.e., a relatively short time interval between adjacent input pulses
- the improved resolution comes at the cost of a shorter range (z-direction) over which a sufficient pulse power is maintained.
- a short PRI is useful, for example, in LIDAR applications where two target vehicles on adjacent lanes are relatively close to each other, but need to be identified as two separate vehicles. If the spatial resolution was too low in this case, the vehicles may appear to be a single large object. It is clear that for a vehicle-based LIDAR system to perform effectively, it needs to be able to adjust the PRI from time to time as traffic conditions change.
- FIG. 2 is a timeline showing an exemplary implementation of varying the PRI of a seed pulse applied to a fiber-based optical amplifier.
- the periods of time ⁇ need not necessarily be fixed; the drawing of FIG. 2 showing equal time periods is for the purposes of illustration only.
- Each time interval represents a different seed pulse PRI, that is, the PRI of the input pulses applied to the EDFA portion of an optical amplifier.
- the process begins with input (seed) pulses defined to exhibit a first PRI, shown as PRI 1 in FIG.
- PRI 1 is a relatively long time interval (e.g., an interval of 50 nsec) between seed pulses.
- This PRI is maintained for the duration of a first period of time ⁇ 1 .
- the PRI is shortened in following time period ⁇ 2 , depicted in FIG. 2 as PRI 2 ⁇ PRI 1 .
- Shortening the PRI means that the time interval between adjacent seed pulses is reduced.
- the following third time period ⁇ 3 is shown in this example as using an extremely short PRI (PRI 3 ⁇ PRI 2 ).
- the PRI during a fourth time period ⁇ 4 is shown as returning to PRI 1 (in this particular example).
- the illustration of various PRIs in FIG. 2 is shown merely as an example of a sequence of different PRIs that may be used control the time interval between the arrival of adjacent input pulses at an EDFA.
- the changes in PRI may be dictated by a system controller, which may quickly change the PRI in applications such as LIDAR as the physical environment changes (e.g., in a vehicle-based LIDAR, the PRI may change as the traffic pattern changes).
- LIDAR physical environment changes
- MOPAs have been found to exhibit transients in the output energy as the PRI changes. The transients are related to the (unwanted) ASE generated within the EDFA as the seed pulses are being amplified.
- the total ASE energy accumulated during a given time interval scales with the PRI; as the time interval between input seed pulses increases (i.e., PRI lengthens), the period of time during which ASE is generated increases as well, thereby increasing the total ASE generated during the time interval (which therefore gains the gain created during this time interval). Conversely, if the PRI is reduced in the length, a lesser amount of ASE is generated between subsequent signal pulses (again changing the gain provided between seed pulses). This variable ASE component thus results in unwanted fluctuations in the gain generated within the EDFA, resulting in creating transients in the energy of the amplifier output pulses.
- FIG. 3 is a block diagram of an exemplary MOPA 10 formed in accordance with one or more embodiments of the present invention to provide transient-free output pulse energy, even under conditions where the PRI of the input seed pulse is frequently changed.
- controlling the parameters of the pump source has been found to allow for fluctuations in ASE to be compensated in a manner where the gain within the EDFA remains constant, and the output pulses thus exhibit a constant (transient-free) output energy level.
- MOPA 10 is shown as comprising an input laser source (seed source) 12 for generating extremely narrow seed pulses (e.g., nsec-scale pulse width) that are then applied as an input pulse signal to a fiber-based optical amplifier, here an erbium doped fiber amplifier (EDFA) 14 .
- a pump source 16 is used to supply the light that stimulates emission from the dopants in the fiber core and amplifies any optical signal passing through the fiber (here, the seed pulses are the signal propagating through the fiber).
- the ultra-short (nsec) seed pulses trigger the generation of high-power pulses as the output from EDFA 14 (i.e., a “pump-and-dump” process) by releasing a given amount of optical energy stored in the doped fiber (the energy created by the presence of the pump light in the doped fiber).
- a pump-and-dump process a given amount of optical energy stored in the doped fiber (the energy created by the presence of the pump light in the doped fiber).
- other rare-earth dopants may be used in the formation of a fiber-based optical amplifier, with erbium only one such option.
- EDFA 14 it is to be understood as also including these various alternative dopant sources.
- a driver circuit 13 provides an input electrical signal to seed laser 12 , where the repetition rate of the current pulses from driver circuit 13 defines the PRI of the seed pulses generated by laser 12 .
- Each pulse itself is relatively narrow (on the order of nsec), with the PRI varying perhaps over the range of about 100 nsec to about 100 ⁇ sec.
- solid-line paths are used to denote optical signal paths and dotted-line paths are used to denote electrical signal paths.
- pump source 16 is configured to provide pulses of pump light as a second input to EDFA 14 and controlled such that a single pump pulse is introduced into EDFA 14 during the PRI.
- Pump source 16 typically includes a laser diode configured to emit radiation at a wavelength known to provide optical amplification in the presence of a rare-earth dopant. When erbium is used as the dopant, a laser diode operating at a wavelength of 980 nm is typically used.
- the constant output energy MOPA of the present invention utilizes pulses of pump light.
- a CW pump results in generating variable amounts of ASE during the time interval between adjacent input seed pulses as the PRI is changed (i.e., a transition between PRIs), creating undesirable transients in the energy of the high power output pulses.
- a pump driver circuit 18 is included in MOPA 10 and utilized to apply a pulsed electrical drive current input to pump source 16 such that parameters of the pump pulse are controlled to create transient-free output pulses.
- Driver circuit 18 is configured to control both the pulse rate and pulse width of pulses of the pump light from source 16 . As discussed in detail below, controlling the repetition rate of the pump pulses, as well as the width of the pump pulse, allows for contribution to the gain from ASE to be managed during PRI transitions such that the output pulse energy remains essentially constant.
- a system controller 20 that is utilized to control the operation of both laser driver 13 and pump driver 18 , ensuring that they each operate with the same PRI. Since the amount of gain achieved within EDFA 14 is a function of the amount of pump light within the doped fiber core, the pump pulses are somewhat longer in duration than the input signal narrow pulse (the pump pulse width on the order of about 25 nsec to a few ⁇ sec, for example, as compared to input seed pulses on the order of tens of nsec). As will be discussed below, system controller 20 is further used to control the width of the pump pulses to equalize the amount of ASE present between each output pulse, even as the PRI changes. Said another way, pump pulses are controlled in both repetition rate and width such that the gain generated in the EDFA is held essentially constant, regardless of changes in the seed pulse PRI.
- the total output energy ⁇ T created by MOPA 10 in response to an input seed pulse can be expressed as follows:
- ⁇ S is the output energy associated with the amplified, high power output pulse P and ⁇ ASE is the (unwanted) ASE noise generated during the same PRI by MOPA 10 .
- ⁇ S is the output energy associated with the amplified, high power output pulse P and ⁇ ASE is the (unwanted) ASE noise generated during the same PRI by MOPA 10 .
- the total output energy created during a given PRI time period can also be expressed as:
- ⁇ T C 1 * ( ⁇ 0 t 1 ⁇ i 1 ⁇ d ⁇ t + ⁇ t 1 t 2 ⁇ i 1 ⁇ d ⁇ t )
- t 1 is a given time duration of an input pump pulse necessary for generating the desired energy of output pulse P for a given pump current value i 1 .
- the interval t 1 to t 2 (defined as ⁇ t) is the time interval attributed to providing the energy to the ASE.
- a longer time interval between seed pulses allows for a rather large amount of ASE to accumulate, when compared to the amount of ASE accumulated during a shorter interval (a “fast PRI”). Transients in terms of the energy within an output pulse thus occur as the PRI changes between each of these time periods.
- the interval ⁇ t needs to scale linearly in accordance with changes in PRI. That is, when the PRI doubles in length, the interval ⁇ t needs to double as well in order to maintain a “constant” accumulation of ASE energy (and thereby maintain a constant gain) during that time interval. Similarly, if the PRI is cut in half, ⁇ t must also decrease by half as much.
- the pulse width of the pump pulse as regulated by controller 20 , thus changes by a predetermined amount in concert with changes in PRI to maintain a constant energy (transient-free) output pulse train.
- FIG. 4 illustrates another embodiment of the present invention, denoted MOPA 10 A, which includes a look-up table 40 that stores a set of a priori time intervals ⁇ t associated with providing specific adjustments in pump pulse width as a function of PRI.
- look-up table 40 may be included as a component within system controller 20 . In this embodiment, therefore, when system controller 20 receives instructions to change to a new PRI, look-up table 40 is utilized to provide the proper pump pulse adjustment ⁇ t required to maintain a constant gain within EDFA 14 .
- system controller 20 provides two inputs to pump driver 18 , a first input defining the PRI and a second input defining the pulse width for that PRI value.
- the operation of laser driver circuit 13 and pump driver circuit 18 are preferably controlled such that seed pulses and pump pulses both exhibit the same PRI.
- the seed pulses and pump pulses are not necessarily synchronized.
- a preferred embodiment of the present invention may be configured such that the pump pulse arrives at EDFA 14 slightly in advance of the seed pulse. By controlling the arrival times of the two pulses, the energy required to “fuel” the seed pulse is delivered “just in time” for that seed pulse and not spread across the entire pulse interval (which is the case for conventional CW pumping in a MOPA).
- FIG. 5 illustrates an alternative embodiment of the present invention, shown as MOPA 10 B, that is configured to time the arrival of the seed pulses with respect to the pump pulses.
- MOPA 10 B of FIG. 3 includes a delay element 50 disposed between controller 20 and laser driver circuit 13 .
- Delay element 50 functions to shift the arrival of the seed pulses until slightly after the arrival of the pump pulses, while maintaining the same PRI for both pulse streams.
- FIG. 6 illustrates an exemplary MOPA 10 C formed in accordance with the present invention that includes a feedback loop between the output of EDFA 14 and pump driver 18 .
- Controller 60 converts the received optical signal into an electrical equivalent that is thereafter analyzed to determine if any adjustments are necessary to pump source 16 (e.g., adjusting the drive current applied to pump source 16 , changing the bias voltage applied pump source 16 , etc.).
- the electrical signal created by pump controller 60 may be applied as an input to system controller 20 that is particularly configured to analyze this feedback and provide the necessary adjustments to the operating parameters of pump source 16 via pump driver 18 .
- the ability to modify pump pulse characteristics allows for the shaded ASE regions to be equalized, regardless of PRI, by adjusting the operating parameters of the pump source.
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Abstract
Description
- The present invention relates to a master oscillator power amplifier (MOPA) and, more particularly, to a fiber-based MOPA configured to provide output pulses of constant energy regardless of changes in the input signal pulse repetition rate.
- A MOPA is known in the art as a specific type of optical amplifier that comprises at least two separate elements, a laser source (the “master oscillator”) and an optical amplifier. At times, the laser source is referred to as the “seed laser”. As this name implies, the laser source is used to “seed” an optical amplifier with an input trigger signal that then generates a high power output signal pulse. By virtue of using a separate power amplification component, the various performance aspects of the laser source itself are decoupled from the requirements of the power generator. Indeed, the MOPA technology provides an efficient power scaling architecture for pulsed laser applications such as “light detection and ranging” (LIDAR). LIDAR is a technology that can be used to measure distances to remote targets, with a laser source used to generate optical pulses that are amplified and directed toward a target which then scatters the light. The separate power amplifier within the MOPA can thus be independently controlled to provide the desired amount of signal gain for a given LIDAR application. Some of the scattered light is received at a detector co-located with the laser source and the distance to the target is then determined based on one or more characteristics of the returned light.
- In many LIDAR applications, a relatively high power (e.g., on the order of hundreds of watts) optical pulse is required so that the scattered, returned light has enough power to yield accurate distance calculations. For some applications (such as, for example, on-board automotive LIDAR systems), the MOPA is configured to generate extremely narrow output pulses and thus requires the use of nsec-scale seed laser pulses. The amplifier portion typically comprises a fiber-based (rare-earth) amplifier (such as an erbium-doped fiber amplifier, EDFA) that utilizes pump light at an appropriate wavelength (e.g., 980 nm) to excite the rare-earth ions in the fiber and thereby amplify the seed laser input signal pulses to a power level sufficient for the required “high power” output pulses. The seed laser is controlled to exhibit a predetermined pulse repetition rate. Instead of defining the input signal pulse train in terms of repetition rate, it is also common to define the pulse train by its “pulse repetition interval” (PRI), which defines the time interval between adjacent pulses (typically measured from the rising edge of a first pulse to the rising edge of a second pulse).
- For applications such as LIDAR, there is a need to vary the PRI over an extended period of time to account for constant changes in the surrounding area being surveyed. Changing the PRI has been found to create a transient change in output energy, which is attributed to changes in the amount of amplified spontaneous emission (ASE), radiative noise, produced as the PRI is changed. Previously, this problem has been addressed by controlling the drive current applied to the pump source so as to modify the amount of pump energy available as a function of changes in PRI. This not considered as a satisfactory solution in many applications, such as MOPAs utilizing multiple PRIs that change rapidly as a function of time, since the gain response of the amplifier may not be managed as quickly as the change in PRI.
- The needs remaining in the prior art are addressed by the present invention, which relates to a master oscillator power amplifier (MOPA) and, more particularly, to a fiber-based MOPA configured to provide high power output pulses of constant energy regardless of changes in the input signal pulse repetition rate (PRI).
- In accordance with the principles of the present invention, a fiber-based MOPA is configured to utilize a pump source that operates in pulse mode (rather than CW, as in the prior art), with the arrival time of the pump pulses coordinated with the arrival time of the input seed pulses. The use of a pulsed pump is described in detail in U.S. Pat. No. 7,100,167, entitled “System and Method for Dynamic Range Extension and Stable Low Power Operation of Optical Amplifiers Using Pump Laser Pulse Modulation”, issued on Sep. 19, 2006 to A. Gurusami et al. and herein incorporated by reference. In addition to the utilization of a pulsed pump as taught by Gurusami et al., the fiber-based MOPA of the present invention also controls the width of the pump pulses (as well as their arrival time), thus providing a mechanism for controlling both the amount of pump energy injected into the amplifier, as well as the overlap in time between the pump pulse and the input pulse. As the PRI of the input signal pulse changes, the timing of the pump pulses and their width are also changed so that a “constant gain” environment is created within the amplifying medium, providing an essentially constant energy output pulse, regulating the amount of ASE generated during different PRIs.
- One embodiment of the present invention takes the form of a MOPA including a section of doped optical fiber for providing signal gain to input light in the presence of a pump light beam operating at a defined wavelength, an input pulse source for generating seed pulses applied as a first input to the section of doped optical fiber, the input pulse source designed to adjust a pulse repetition interval (PRI) between adjacent pulses in response to a “PRI change” control signal, an a pump source for generating pump pulses applied as a second input to the section of doped optical fiber. The presence of the seed and pump pulses within the section of doped optical fiber amplifies the power of the seed pulses and generates high-power optical output pulses. The pump pulse source itself is designed to control pump pulse parameters (including repetition rate and pulse width) to maintain an essentially constant energy in the high-power optical output pulses regardless of the seed pulse PRI.
- Other and further aspects and principles of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
- Referring now to the drawings,
-
FIG. 1 is a simplified diagram of a prior art LIDAR system, useful for understanding an exemplary implementation of the constant pulse energy MOPA of the present invention; -
FIG. 2 is a diagram illustrating a sequence of four different PRIs, utilized in sequence to control a pulsed laser in an exemplary MOPA; -
FIG. 3 illustrates an exemplary constant energy MOPA formed in accordance with the present invention; -
FIG. 4 illustrates an alternative embodiment of a MOPA formed in accordance with the present invention, illustrating a look-up table used to correlate changes in PRI with pump pulse adjustments to maintain constant gain; -
FIG. 5 shows yet another embodiment of a MOPA formed in accordance with the present invention, in this case including a delay element to control the arrival times of seed pulses and pump pulses at the fiber amplifier; and -
FIG. 6 illustrates an embodiment of the present invention where the inventive MOPA further comprises a feedback path between the fiber amplifier output and the pump source input. - Prior to describing the details of a MOPA formed in accordance with the principles of the present invention, a typical utilization of a MOPA as a component in a LIDAR system will be reviewed, providing a context for understanding the details of the principles of the present invention.
FIG. 1 illustrates anexemplary LIDAR system 1 used to measure the distance D betweensystem 1 and a target TAR. In the particular example of a vehicle-based LIDAR system, the target may be a second vehicle traveling down a road in front of the vehicle equipped with LIDARsystem 1.System 1 includes alight source 2 that generates high-powerlight signal pulses 3,pulses 3 being of a predetermined wavelength λsig and separated in time by a predetermined “pulse repetition interval”, or simply PRI. Pulses 3 pass through abeam splitter 4 andexit system 1. Aspulses 3 reach target TAR, some of the reflected/scattered light is directed back towardLIDAR system 1 as returnedpulses 5. Returnedpulses 5 are thereafter re-directed bybeam splitter 4 into areceiver 6 that is configured to perform a programmed analysis of the pulses to determine, for example, the distance D betweenLIDAR system 1 and target TAR. - In many applications, there is a need to vary the PRI of
pulses 3exiting source 1. For example, a “long” PRI (i.e., a relatively long time between adjacent pulses) allows for long-range sensing. In LIDAR applications, a long PRI allows the system to “see” vehicles far ahead or behind and make appropriate decisions in a timely fashion (particularly required in dynamic situations where movement of targets occurs). The advantages of using a long PRI come at the cost of reduced spatial resolution in the x-y plane of the pulse (seeFIG. 1 ). On the other hand, a “short” PRI (i.e., a relatively short time interval between adjacent input pulses) provides a higher degree of spatial resolution. The improved resolution comes at the cost of a shorter range (z-direction) over which a sufficient pulse power is maintained. A short PRI is useful, for example, in LIDAR applications where two target vehicles on adjacent lanes are relatively close to each other, but need to be identified as two separate vehicles. If the spatial resolution was too low in this case, the vehicles may appear to be a single large object. It is clear that for a vehicle-based LIDAR system to perform effectively, it needs to be able to adjust the PRI from time to time as traffic conditions change. -
FIG. 2 is a timeline showing an exemplary implementation of varying the PRI of a seed pulse applied to a fiber-based optical amplifier. In this particular case, each PRI is sequentially utilized over fixed periods of time τ (shown as τ=X00 μsec inFIG. 2 ). It is to be noted that the periods of time τ need not necessarily be fixed; the drawing ofFIG. 2 showing equal time periods is for the purposes of illustration only. Each time interval represents a different seed pulse PRI, that is, the PRI of the input pulses applied to the EDFA portion of an optical amplifier. In the timeline ofFIG. 2 , the process begins with input (seed) pulses defined to exhibit a first PRI, shown as PRI1 inFIG. 2 , where for the purposes of discussion it is presumed that PRI1 is a relatively long time interval (e.g., an interval of 50 nsec) between seed pulses. This PRI is maintained for the duration of a first period of time τ1. Subsequently, the PRI is shortened in following time period τ2, depicted inFIG. 2 as PRI2<PRI1. Shortening the PRI means that the time interval between adjacent seed pulses is reduced. The following third time period τ3 is shown in this example as using an extremely short PRI (PRI3<PRI2). The PRI during a fourth time period τ4 is shown as returning to PRI1 (in this particular example). The illustration of various PRIs inFIG. 2 is shown merely as an example of a sequence of different PRIs that may be used control the time interval between the arrival of adjacent input pulses at an EDFA. - The changes in PRI may be dictated by a system controller, which may quickly change the PRI in applications such as LIDAR as the physical environment changes (e.g., in a vehicle-based LIDAR, the PRI may change as the traffic pattern changes). As mentioned above, conventional MOPAs have been found to exhibit transients in the output energy as the PRI changes. The transients are related to the (unwanted) ASE generated within the EDFA as the seed pulses are being amplified. In particular, it has been found that the total ASE energy accumulated during a given time interval scales with the PRI; as the time interval between input seed pulses increases (i.e., PRI lengthens), the period of time during which ASE is generated increases as well, thereby increasing the total ASE generated during the time interval (which therefore gains the gain created during this time interval). Conversely, if the PRI is reduced in the length, a lesser amount of ASE is generated between subsequent signal pulses (again changing the gain provided between seed pulses). This variable ASE component thus results in unwanted fluctuations in the gain generated within the EDFA, resulting in creating transients in the energy of the amplifier output pulses.
-
FIG. 3 is a block diagram of an exemplary MOPA 10 formed in accordance with one or more embodiments of the present invention to provide transient-free output pulse energy, even under conditions where the PRI of the input seed pulse is frequently changed. As will be described below in association withFIGS. 3-6 , controlling the parameters of the pump source has been found to allow for fluctuations in ASE to be compensated in a manner where the gain within the EDFA remains constant, and the output pulses thus exhibit a constant (transient-free) output energy level. Referring in particular toFIG. 3 , MOPA 10 is shown as comprising an input laser source (seed source) 12 for generating extremely narrow seed pulses (e.g., nsec-scale pulse width) that are then applied as an input pulse signal to a fiber-based optical amplifier, here an erbium doped fiber amplifier (EDFA) 14. Apump source 16 is used to supply the light that stimulates emission from the dopants in the fiber core and amplifies any optical signal passing through the fiber (here, the seed pulses are the signal propagating through the fiber). In particular, the ultra-short (nsec) seed pulses trigger the generation of high-power pulses as the output from EDFA 14 (i.e., a “pump-and-dump” process) by releasing a given amount of optical energy stored in the doped fiber (the energy created by the presence of the pump light in the doped fiber). It is to be understood that other rare-earth dopants may be used in the formation of a fiber-based optical amplifier, with erbium only one such option. Thus, while the following description refers to “EDFA 14”, it is to be understood as also including these various alternative dopant sources. - A
driver circuit 13 provides an input electrical signal to seedlaser 12, where the repetition rate of the current pulses fromdriver circuit 13 defines the PRI of the seed pulses generated bylaser 12. Each pulse itself is relatively narrow (on the order of nsec), with the PRI varying perhaps over the range of about 100 nsec to about 100 μsec. In the illustration ofFIG. 3 , solid-line paths are used to denote optical signal paths and dotted-line paths are used to denote electrical signal paths. - In accordance with the principles of the present invention, pump
source 16 is configured to provide pulses of pump light as a second input toEDFA 14 and controlled such that a single pump pulse is introduced intoEDFA 14 during the PRI. Pumpsource 16 typically includes a laser diode configured to emit radiation at a wavelength known to provide optical amplification in the presence of a rare-earth dopant. When erbium is used as the dopant, a laser diode operating at a wavelength of 980 nm is typically used. In contrast to many prior art EDFAs that utilize a continuous wave (CW) pump, the constant output energy MOPA of the present invention utilizes pulses of pump light. As mentioned above, it has been found in the past that when the PRI is varied, a CW pump results in generating variable amounts of ASE during the time interval between adjacent input seed pulses as the PRI is changed (i.e., a transition between PRIs), creating undesirable transients in the energy of the high power output pulses. - In accordance with the principles of the present invention, a
pump driver circuit 18 is included in MOPA 10 and utilized to apply a pulsed electrical drive current input to pumpsource 16 such that parameters of the pump pulse are controlled to create transient-free output pulses.Driver circuit 18 is configured to control both the pulse rate and pulse width of pulses of the pump light fromsource 16. As discussed in detail below, controlling the repetition rate of the pump pulses, as well as the width of the pump pulse, allows for contribution to the gain from ASE to be managed during PRI transitions such that the output pulse energy remains essentially constant. - Also shown in
FIG. 3 is asystem controller 20 that is utilized to control the operation of bothlaser driver 13 andpump driver 18, ensuring that they each operate with the same PRI. Since the amount of gain achieved withinEDFA 14 is a function of the amount of pump light within the doped fiber core, the pump pulses are somewhat longer in duration than the input signal narrow pulse (the pump pulse width on the order of about 25 nsec to a few μsec, for example, as compared to input seed pulses on the order of tens of nsec). As will be discussed below,system controller 20 is further used to control the width of the pump pulses to equalize the amount of ASE present between each output pulse, even as the PRI changes. Said another way, pump pulses are controlled in both repetition rate and width such that the gain generated in the EDFA is held essentially constant, regardless of changes in the seed pulse PRI. - The total output energy εT created by MOPA 10 in response to an input seed pulse can be expressed as follows:
-
εT=εS+εASE, - where εS is the output energy associated with the amplified, high power output pulse P and εASE is the (unwanted) ASE noise generated during the same PRI by MOPA 10. For the purposes of the present invention, the total output energy created during a given PRI time period can also be expressed as:
-
- where t=0 is defined as the beginning (trigger) for the seed pulse applied as an input to the EDFA, and t1 is a given time duration of an input pump pulse necessary for generating the desired energy of output pulse P for a given pump current value i1. The interval t1 to t2 (defined as Δt) is the time interval attributed to providing the energy to the ASE.
- Without any type of ASE compensation, a longer time interval between seed pulses (a “slow PRI”) allows for a rather large amount of ASE to accumulate, when compared to the amount of ASE accumulated during a shorter interval (a “fast PRI”). Transients in terms of the energy within an output pulse thus occur as the PRI changes between each of these time periods.
- In accordance with the principles of the present invention, assuming that the amount of ASE generated remains constant during the operation of EDFA, the interval Δt needs to scale linearly in accordance with changes in PRI. That is, when the PRI doubles in length, the interval Δt needs to double as well in order to maintain a “constant” accumulation of ASE energy (and thereby maintain a constant gain) during that time interval. Similarly, if the PRI is cut in half, Δt must also decrease by half as much. The pulse width of the pump pulse, as regulated by
controller 20, thus changes by a predetermined amount in concert with changes in PRI to maintain a constant energy (transient-free) output pulse train. -
FIG. 4 illustrates another embodiment of the present invention, denotedMOPA 10A, which includes a look-up table 40 that stores a set of a priori time intervals Δt associated with providing specific adjustments in pump pulse width as a function of PRI. In one implementation, look-up table 40 may be included as a component withinsystem controller 20. In this embodiment, therefore, whensystem controller 20 receives instructions to change to a new PRI, look-up table 40 is utilized to provide the proper pump pulse adjustment Δt required to maintain a constant gain withinEDFA 14. Thus, in the embodiment ofFIG. 4 ,system controller 20 provides two inputs to pumpdriver 18, a first input defining the PRI and a second input defining the pulse width for that PRI value. - As mentioned above, the operation of
laser driver circuit 13 andpump driver circuit 18 are preferably controlled such that seed pulses and pump pulses both exhibit the same PRI. It is to be noted that the seed pulses and pump pulses are not necessarily synchronized. In fact, a preferred embodiment of the present invention may be configured such that the pump pulse arrives atEDFA 14 slightly in advance of the seed pulse. By controlling the arrival times of the two pulses, the energy required to “fuel” the seed pulse is delivered “just in time” for that seed pulse and not spread across the entire pulse interval (which is the case for conventional CW pumping in a MOPA). -
FIG. 5 illustrates an alternative embodiment of the present invention, shown asMOPA 10B, that is configured to time the arrival of the seed pulses with respect to the pump pulses. Besides the components discussed above in accordance with the embodiment ofFIG. 3 ,MOPA 10B ofFIG. 3 includes adelay element 50 disposed betweencontroller 20 andlaser driver circuit 13. Delayelement 50 functions to shift the arrival of the seed pulses until slightly after the arrival of the pump pulses, while maintaining the same PRI for both pulse streams. - There are a variety of applications where a MOPA is subjected to environmental changes (such as changes in ambient temperature) that impact the performance of the amplifier. As the components age, their performance may also be impacted.
FIG. 6 illustrates anexemplary MOPA 10C formed in accordance with the present invention that includes a feedback loop between the output ofEDFA 14 andpump driver 18. In operation, a portion of the amplified, high power pulses P is coupled into a feedback optical signal path and applied as an input to pumpcontroller 60.Controller 60 converts the received optical signal into an electrical equivalent that is thereafter analyzed to determine if any adjustments are necessary to pump source 16 (e.g., adjusting the drive current applied to pumpsource 16, changing the bias voltage appliedpump source 16, etc.). In an alterative embodiment, the electrical signal created bypump controller 60 may be applied as an input tosystem controller 20 that is particularly configured to analyze this feedback and provide the necessary adjustments to the operating parameters ofpump source 16 viapump driver 18. - Thus, in accordance with the present invention, the ability to modify pump pulse characteristics allows for the shaded ASE regions to be equalized, regardless of PRI, by adjusting the operating parameters of the pump source. Various additional modifications of this invention will occur to those skilled in the art. All deviations from the specific teachings of this specification that basically rely on the principles and their equivalents through which the art has been advanced are properly considered to be within the scope of the invention as described and claimed.
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