Systems and methods for measuring ultra-short light pulses

Systems and methods for measuring a pulse length (τ0) of an ultra-short light pulse (P0) based on processing a number of substantially similar light pulses. The system includes an autocorrelation optical system adapted to receive the light pulses P0 and create from each light pulse two beams having an associated optical path length difference ΔOPL. Providing a different ΔOPL for each light pulse creates an autocorrelation interference pattern representative of an autocorrelation of the light pulse P0. An LED detector detects the autocorrelation interference pattern and generates therefrom an autocorrelation signal. A signal-processing unit forms from the autocorrelation signal a digital count signal representative of a number of counted peaks in the autocorrelation signal above the full-width half maximum. Control electronics unit causes the varying ΔOPL and provides a difference signal (SΔ) representative of the ΔOPL to the signal-processing unit. The signal processing unit is adapted to calculate the pulse length based on ΔOPL and the number NC of counted peaks. The autocorrelation optical system can either be prism-based or electro-optical-interferometer based. The measurement system can be made very compact and is far less expensive and far easier to use as compared to prior art ultra-short pulse measurement systems.

FIELD OF THE INVENTION

The present invention relates generally to measuring light pulses, and particularly to systems and methods for such measuring ultra-short laser light pulses using autocorrelation techniques.

BACKGROUND ART

Light sources such as lasers that generate “ultra-short” light pulses are becoming more commonplace in high-tech industries as new methods are being developed to utilize their characteristics. “Ultra-fast” and “ultra-short” typically refer to the temporal duration (length) of a light pulse, and in particular to pulses having durations less than a few hundred femtoseconds (fs). Present-day lasers are capable of producing light pulses of 50-100 fs and as short as 6 fs. Future lasers will be capable of producing even shorter light pulses.

For such ultra-short pulses, a unique advantage is their extremely high energies over ultra-short time scales. Such pulses allow access to unique physical processes that only occur at these energies and time scales, such as laser micromachining and certain biological and medical applications (e.g., laser in-situ keratomileusis (LASIK)). Knowing the pulse irradiance (measured in watts per unit area) with a high degree of accuracy is critical for most processes that employ ultra-short light pulses. In order to quantify the irradiance of ultra-short light pulses, one must know their exact temporal width.

Because ultra-short light pulses exist for extremely short time periods by definition, there is no direct way to measure their width. This is largely because most atoms and materials do not react sufficiently fast. Accordingly, the state-of-the-art measurement techniques for ultra-short pulses rely on measuring a non-linear effect caused by an ultra-short pulse and then backing out the pulse length.

The most common technique for measuring ultra-short light pulses involves splitting the incident pulse (beam) inside a Michelson-style interferometer, with one interferometer arm sweeping back and forth. The distance the arm must sweep equates to the physical length of the laser pulse (e.g., a 100 fs pulse represents a length of 30 μm). The interferometer provides the autocorrelation of the pulse, which is measured using an oscilloscope. However, a Michelson interferometer is very sensitive to alignment so it typically takes significant time to set up and maintain it in proper operating condition for making measurements.

Another pulse measuring technique called Frequency Resolved Optical Gating (FROG) relies on splitting the incident beam into two separate beams and recombining the separated beams inside a non-linear crystal. Four-wave mixing occurs inside the crystal and a new beam is generated that has double the optical frequency of the input beam. The new beam is recorded via a detector (e.g., a charge-coupled device or “CCD”), which provides information about the frequency and temporal information of the new beam shape. When viewing the beam profile using the FROG technique, one axis represents the spectrum of the pulse (which is relatively wide since the pulse is relatively short), while the other axis represents the temporal shape. While the FROG technique is very convenient, it requires that the incident beam have a perfect Gaussian profile. Most lasers, however, do not have the requisite idealized Gaussian profile, so that the time and spectrum measurements tend to be inaccurate.

More recently, a pulse-width measuring technique was developed by Reid et al., and described in the article by Reid et al., entitled “Light-emitting diodes as measurement devices for femtosecond laser pulses,”Optics Letters, Vol. 22, No. 4, published on Feb. 15, 1997 (hereinafter, “the Reid article”), which article is incorporated by reference herein. The Reid article device utilizes a movable prism and an unbiased LED that has a non-linear power-dependent response. The prism splits an initial laser beam into two shifted beams that interfere. Moving the prism for each new incident pulse causes an autocorrelation interference pattern to sweep across the LED detector, which generates a corresponding autocorrelation signal.

Though various approaches to ultra-short pulse measurement are embodied in a number of different commercial devices, these devices have significant shortcomings. One serious shortcoming is that they are unusually difficult to use in practice mostly because they are difficult to align. This lack of functionality is particularly problematic given that frequent system alignment is needed for most light-pulse-measurement applications. Further, the measurement devices are subject to beam-shape limitations—that is to say, poor-quality beam shapes result in poor measurements. In addition, the typical ultra-short pulse measurement device is very costly—about $20,000 or more in present-day dollars.

Accordingly, efficient, cost-effective and commercially viable systems and methods for measuring ultra-short light pulses are needed.

SUMMARY OF THE INVENTION

One aspect of the invention is a system for measuring the temporal pulse length (τ0) (also referred to as the “pulse width”) of an ultra-short light pulse. The system includes an autocorrelation optical system adapted to receive the light pulse and create therefrom two beams having an associated optical path length difference ΔOPL that varies to form an autocorrelation interference pattern representative of an autocorrelation of the light pulse. The autocorrelation optical system can be prism-based wherein a movable Wollaston prism is used to vary ΔOPL. The autocorrelation optical system can also be interferometer-based, wherein electro-optical (EO) elements in one or both interferometer arms are used to vary ΔOPL by applying a variable voltage to the EO elements. An LED detector is arranged to detect the autocorrelation interference pattern that sweeps across the detector as ΔOPL is changed for each new light pulse P0. The LED detector is adapted to generate therefrom a raw autocorrelation signal based on the system processing a number of substantially similar light pulses P0each with a different value for ΔOPL. A signal-processing unit is electrically coupled to the LED detector and adapted to condition the raw autocorrelation signal and form from the conditioned signal a digital count signal representative of a number of counted peaks in the autocorrelation signals that are above the full-width half-max (FWHM) of the raw autocorrelation signal. A control electronics unit is operably coupled to the autocorrelation optical system and the signal-processing unit and is adapted to cause the varying ΔOPL that forms the autocorrelation interference pattern. The control electronics unit also provides a difference signal (SΔ) representative of the ΔOPL to the signal-processing unit. The signal-processing unit is adapted to calculate the pulse length based on ΔOPL and the number NCof counted autocorrelation signal peaks.

Another aspect of the invention is a method of measuring the temporal pulse length of an ultra-short light pulse using a number of substantially similar input light pulses. The method includes dividing each light pulse into two co-polarized light beams having an associated optical path length difference ΔOPL, providing a different value of ΔOPL for each light pulse, and interfering the two light beams associated with each light pulse to form an autocorrelation interference pattern representative of the autocorrelation of the light pulse. The method also includes using an LED detector to convert the autocorrelation interference pattern into an autocorrelation signal. The method further includes measuring the autocorrelation signal to determine a full-width half-max (FWHM) threshold LTHof the autocorrelation signal. The method also includes using a programmable comparator and a digital logic counter to count the number NCof peaks in the autocorrelation signal that fall above the signal's FWHM based on threshold LTH, and then forming a digital count signal representative of the number NCof counted peaks. The method also includes using a microcontroller to calculate the pulse length based on ΔOPL and NC.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to system and methods for measuring the (temporal) pulse length of ultra-short light pulses such as created by ultra-fast lasers. The systems and methods of the present invention are particularly suited for forming a commercially viable ultra-short pulse measurement device.

An overview of the light-pulse measurement system is first provided, followed by details of an example measurement system having a prism-based autocorrelator optical system. Example light-pulse measurement systems based on several different EO-based autocorrelator optical systems are then described. The benefits of the present invention in terms of system alignment and pulse-width accuracy are then discussed.

In the description below, reference is made to an “autocorrelation signal” SAthat may be, for example, a “raw” autocorrelation signal SARor a conditioned autocorrelation signal SACthat reflects a particular processing state of the autocorrelation signal. Reference to autocorrelation signal SAis thus general and is not limited to a particular processing state, as will be understood by one skilled in the art in the context in which the phrase is used.

Also in the description below, the systems and methods of the present invention are based on measuring a number of different substantially similar light pulses P0. Thus, the invention is described in connection with measuring the temporal pulse length τ0of a light pulses P0, which pulse length is representative of the average pulse width of those light pulses P0used to make the measurement. For the sake of discussion, light pulse P0is referred to in the singular where convenient.

I. Light-Pulse Measurement System Overview

FIG. 1is a schematic diagram of a generalized embodiment of a light-pulse measurement system10of the present invention. Cartesian coordinates12are shown for the sake of reference, with the Y direction being “vertical,” the X direction being “horizontal” and into the paper, and the Z direction also being horizontal and in the plane of the paper.

System10includes, along an optical axis A1, an autocorrelator optical system20adapted to receive an incident light pulse P0of temporal pulse length τ0in a beam B0and generate therefrom (and output) two interfering beams BAand BBthat create an interference pattern representative of an autocorrelation of incident light pulse P0. In an example embodiment, pulse length τ0is measured at the full-width half-max (FWHM) of the light pulse. In an example embodiment, incident light pulse P0is an ultra-short laser pulse emitted by a light source22that is or includes a laser.

In an example embodiment, system10optionally includes an optical coupling system24adapted to optically couple light source22to autocorrelator optical system20.

In order to efficiently measure the pulse length τ0, system10also has an electronics unit30adapted to receive and detect interfered beams BAand BBfor each light inputted light pulse P0, and generate and process the associated electrical autocorrelation signal SAformed by processing a number of inputted light pulses P0each having a different associated value for ΔOPL. In an example embodiment, electronics unit30includes a light-emitting diode (LED) detector32supported on a movable stage33, a signal-processing unit34electrically coupled to the LED detector and the moveable stage, control electronics unit38electrically coupled to the signal-processing unit, and a display unit40electrically coupled to the signal-processing unit. Electronics unit30is electrically connected to autocorrelation optical system20via an electrical connection42that may include a plurality of electrical lines. A power supply44is operably connected to electronics unit30and provides the electrical power for system10.

Typically, light-pulse measurement systems use very thin non-linear crystals and photomultiplier tubes (PMT's) to record the autocorrelation signal. However, non-linear crystals and PMT's are very expensive and easily damaged. On the other hand, LED detector32is or otherwise includes a simple LED in which a non-linear process occurs. In order for this process to occur, LED detector32is chosen to match the photon energy of the light incident thereon. Specifically, the LED bandgap should be close to but greater than the photon energy. Thus, when the intensity of the light incident LED detector32is sufficiently high, multi-photon absorption occurs and the autocorrelation signal SAis generated.

The operation of electronics unit30is discussed in greater detail below in connection with the various example measurement system embodiments set forth below.

In an example embodiment, autocorrelator optical system20is prism-based and includes, in order along optical axis A1(which is oriented along the Z-direction), a half-wave plate50, a first lens60, and a prism unit64. Prism unit64includes a Wollaston prism66formed by two prisms67A and67B with a prism interface68therebetween. Prism unit64is supported by a movable stage70that is operably coupled to prism driver38of electronics unit30. System20further includes a second lens72arranged along optical axis A1and downstream of prism unit64, and a 45° linear polarizer76arranged along optical axis A1and downstream of the second lens. LED detector32of electronics unit30is arranged at the focus of second lens72.

In operation of system20, a linearly-polarized incident light pulse P0having an associated light beam path B0encounters half-wave plate50, which rotates the polarization of the incident light pulse by 45°. Incident light pulse P0is then focused by first lens60and travels to prism unit64and to Wollaston prism66located therein. A suitable prism66is available from Zeta International Corp. of Mt. Prospect, Ill. and costs about $440 for a 10 mm clear-aperture, 5° wedge-angle air-spaced version. Incident light pulse P0is focused by first lens60at prism interface68. Because prisms67A and67B have orthogonal optic axes associated with an extraordinary index of refraction neand an ordinary index of refraction no, Wollaston prism66forms from incident light beam B0two orthogonally polarized light beams: a horizontally polarized light beam BHthat includes an associated horizontally polarized light pulse PH) and a vertically polarized light beam BVthat includes an associated vertically polarized light pulse PV. The angular separation of light beams BHand BVis called the splitting angle α and is given by the relation:
α=2(ne−no)tan θ  Equation (1)
where θ is the prism wedge angle.

One of the light beams (BH) travels a longer optical path length inside prism67B than the other light beam. Accordingly, by slowly moving prism unit64back and forth in the Y-direction over a range90the optical path length (OPL) experienced by the two beams BHand BVfor each incident light pulse P0changes with time, i.e., the optical path length difference ΔOPL between these beams changes with each incident light pulse. This leads to the autocorrelation of the incident pulse P0when the light beams are made to overlap (interfere). In an example embodiment, control electronics unit38of electronics unit30is (or includes) a prism driver that controls the movement of prism unit64via control signal S42sent to movable stage70via electrical link42, as described below so that a different value for ΔOPL occur for each incident light pulse P0.

In order to detect an autocorrelation signal via the movement of Wollaston prism66, light beams BHand BVare focused by second lens72through 45° linear polarizer76. This makes the two beams co-polarized at 45° polarization so that they are able to interfere when focused onto LED detector32. Thus, upon passing through 45° polarizer76, light beams BHand BVbecome co-polarized beams BAand BB, respectively. The interference pattern formed on LED detector32by the interference of beams BAand BBis converted into the aforementioned (analog) electrical signal SArepresentative of the autocorrelation of input pulse P0. Signal SAis hereinafter generally referred to as the “autocorrelation signal.”

FIG. 3plots the intensity (arbitrary units) vs. time (second) for an example autocorrelation signal SAobtained as described above. The overall envelope of the autocorrelation signal results from the low electrical load on LED detector32. Signal SAalso includes a number of peaks100. The modulation of autocorrelation signal SAis the correlation of the electric field of light pulse P0with itself, and with the modulation spacing (i.e., the distance between peaks100) depending on the movement speed of prism unit64.

The conventional approach to measuring the pulse length τ0of light pulse P0is to connect LED detector32to an oscilloscope and record signal SA. Signal SAis then formatted and plotted, and the number of peaks100over a specified range is counted by hand. From this data, the Full-Width Half-Maximum (FWHM) pulse length τ0can be determined. However, this is a very tedious and inefficient process to carry out, particularly while trying to make adjustments to the light source being measured, such as when the light source is a laser and adjusting the laser cavity. The present invention can be automated to provide a quick and accurate pulse length calculation.

b) Electronics Unit

FIG. 4is a detailed schematic diagram of an example embodiment of electronics unit30suitable for use with a prism-based autocorrelation optical system20. As discussed above, electronics unit30includes LED detector32. In an example embodiment, LED detector32is, for example, an AlGaAs LED, which is available from local electronics stores and that costs about $2.00. Electronics unit30also includes signal-processing unit34. In this prism-based embodiment, control electronics unit38is a prism driver. In an example embodiment, signal-processing unit34includes signal-conditioning electronics190electrically coupled to LED detector32and adapted to condition the raw autocorrelation signal SARgenerated thereby prior to performing signal processing. In an example embodiment, signal processing electronics190includes an amplifier192that amplifies the relatively weak (e.g., on the order 60 to 200 millivolts) autocorrelation signal SA, and an electrical filter194that filters out noise from the amplified signal SA, thereby forming a conditioned autocorrelation signal SAC.

Signal-processing unit34further includes a first analog buffer200electrically connected to the output side of signal-conditioning electronics190. Analog buffer200is adapted to store the (conditioned) analog autocorrelation signal SAand to isolate LED detector32from the other electronic elements, whose load can distort the autocorrelation signal. The output of analog buffer200is electrically connected to the input side of a programmable comparator210.

Signal-processing unit34also includes a reference input unit220that includes a second analog buffer200electrically connected at its input side to the output side of signal-conditioning electronics190and electrically connected at its output side to a FWRM tracking circuit226. FWHM tracking circuit226is, in turn, electrically connected at the output side to the input side of comparator210. FWHM tracking circuit226is adapted to determine the FWHM of autocorrelation signal SA(e.g., the conditioned autocorrelation signal SAC) and to provide a reference signal SRcorresponding thereto.

Reference input unit220is also operably coupled to a user input device230(e.g., a programmable signal generator) adapted to allow a user to manually input a reference signal SR. In an example embodiment, user input is used to tune FWHM tracking circuit226, e.g., via a user input signal SI from user input device230.

Signal-processing unit34further includes a digital logic counter260electrically coupled at its input side to comparator210, and electrically coupled at its output side to a microcontroller270. Prism driver38and display40are also electrically coupled to microcontroller270, as is user input device230. In an example embodiment, microcontroller270is one of the BASIC Stamp® microcontrollers available from Parallax, Inc., Rocklin, Calif., for about $80. Microcontroller270is adapted (e.g., programmed) to carry out the method of operation of system10as described herein.

c) Method of Operation

The method of operation of electronics unit30in prism-based measurement system10is now described. First, the raw autocorrelation signal SARis created from a number of input pulses P0by prism-based autocorrelation optical system20as described above. This raw autocorrelation signal SARthen proceeds from LED detector32to signal-conditioning electronics190, where the autocorrelation signal is conditioned (e.g., amplified and filtered) to form conditioned autocorrelation signal SAC. The conditioned autocorrelation signal SACthen travels to and is temporality stored in first analog buffer200.

In an example embodiment where reference signal SRis generated automatically, conditioned autocorrelation signal SACis also stored in second analog buffer200in reference input unit220. FWHM tracking circuit226in reference input unit220analyzes the autocorrelation signal from second analog buffer200and determines the threshold level LTHcorresponding to the FWHM of signal SAC. FWHM tracking circuit226then generates a reference signal SRrepresentative of the FWHM threshold level LTHas illustrated inFIG. 3. This is done because determining the pulse length τ0of input pulse P0only requires counting peaks100in the autocorrelation signal that are above its FWHM.

Reference signal SRis provided as an input to comparator210, which sets its threshold level to correspond to threshold level LTHso that it only those peaks in the (conditioned) autocorrelation signal above the FWHM are counted. Thus, comparator210compares autocorrelation signal SACto threshold signal SRand generates a digital autocorrelation signal SADhaving digital logic levels for the portion of the signal above the FWHM. Digital autocorrelation signal SADproceeds to digital logic counter260, which is adapted to count the peaks100in this signal and generate a corresponding digital count signal SNrepresentative of the number of counted peaks (“counts”) NC. Digital count signal SNis then provided to microcontroller270for processing.

Meanwhile, microcontroller270activates prism driver38via a signal S38, which generates a stage control signal S42. Stage control signal S42causes stage70to move in the Y-direction, along with prism unit64supported thereby, as indicated by double-arrow90. In this example embodiment, prism driver38also generates a “difference” signal SΔrepresentative of the relative position of prism unit64relative to optical axis A1, and transmits this signal to microcontroller270. Signal SΔis called a “difference” signal because it is representative of the optical path length difference ΔOPL between beams B1and B2. This allows prism unit64to be moved into position for each new input pulse P0.

Microcontroller270is adapted (e.g., programmed) to calculate the pulse length τ0of input pulse P0based on the position of prism unit64(and thus optical path length difference ΔOPL) as represented by difference signals SΔ, and the number of peaks NCin the autocorrelation signal as represented by digital count signal SN. This calculation is described below. After each pass of prism unit64over a select translation movement range90, in an example embodiment microcontroller270generates a calculated pulse-length signal SPLrepresentative of pulse length τ0and sends this signal to display40to display the calculated pulse length. Microcontroller270also sends a reset signal SRESto digital logic counter260that resets the digital logic counter to zero after each pass of prism unit64over the select translation range90so that another pulse-length measurement can be made.

d) Pulse Length Calculation

To calculate the pulse length τ0for the prism-based autocorrelator system20, the prism wedge angle θ and the extraordinary and ordinary indices refraction neand noof the prism must be known. Example values for these parameters are θ˜23°, ne=1.547 and no=1.538, which per equation (1) yield a splitting angle α=0.50°.

In an example embodiment, an estimate of the range for pulse length τ0is inputted (e.g., via user input unit230) into microcontroller270. This narrows the needed travel range90of Wollaston prism66, which in turn narrows the range of autocorrelation signal SA.

For short pulses P0, only a small travel distance d within range90is required because the autocorrelation trace is small. By moving prism unit64over a range90greater than necessary, the possibility of reading noise increases.

The travel distance d of prism unit64is in the Y direction and is given by the equation

d=c·(τe)2·(ne-no)⁢tan⁢⁢θEquation⁢⁢(2)
where c is the speed of light in vacuum and τeis the expected pulse length. This distance is the deviation from the center position of the prism on axis A1.

Once travel distance d is known, microprocessor270directs prism driver38to move prism unit64over the corresponding travel range90as described above. As prism unit64travels, signal-processing unit34digitizes and counts the peaks100in the autocorrelation signal as described above.

The pulse length τ0is calculated via the equation

τ0=NC·λγ·cEquation⁢⁢(3)
where λ the wavelength of the input light pulse P0, c is the speed of light in a vacuum, and γ is a beam-profile parameter that described the profile of the input beam B0. For Gaussian-profile input pulses P0, γ is ˜1.414 and for sech2-profile input pulses, γ˜1.543. These numbers come from the shape of the pulse and their autocorrelation.

In addition, the light propagating in autocorrelator optical system20passes through optical material in prism unit64, first and second lenses60and72, half-wave plate50and 45° polarizer76. This causes input pulse P0to stretch prior to its width τ0being measured. The amount of stretch depends on the second and third terms of chromatic dispersion of the optical material through which the input pulse passes. The “stretched” pulse length τchirpis give by

An example embodiment of the present invention is an electro-optics (EO)-based light-pulse measurement system10. Several example embodiments of EO light-pulse measurement systems10are described below.

FIG. 5is a schematic diagram of an example embodiment of a bulk-optics EO autocorrelator optical system20. System20includes a Mach-Zehnder (MZ) interferometer300having a first 50-50 beamsplitter304A that forms a second optical axis A2that starts out orthogonal to optical axis A1. A first mirror306A is located along second optical axis A2so as to fold this optical axis to bring it parallel to optical axis A1. A second mirror306B is located along optical axis A1downstream of first beamsplitter304A. This second mirror folds optical axis A1so that it intersects optical axis A2. A second 50-50 beamsplitter304B is arranged at the intersection of axes A1and A2, and serves to re-combine axis A2with axis A1. LED detector32is arranged immediately downstream of this second beamsplitter304B along the re-formed optical axis A1.

The optical path associated with optical axis A1between the two beamsplitters defines a first interferometer arm320A having an optical path length OPLAthat carries beam BA. Likewise, the optical path associated with optical axis A2between the two beamsplitters defines a second interferometer arm320B having an optical path length OPLBthat carries beam BB. The optical path length difference between the two interferometer arms is thus defined as ΔOPL=(OPLA−OPLB).

EO autocorrelator optical system20includes a first EO unit328A arranged in first interferometer arm320A, and a second EO unit328B arranged in second interferometer arm320B. In an example embodiment, EO unit328A includes at least one EO member330A, while EO unit330B includes at least one EO member330B. In an example embodiment, EO members330A and330B are EO crystals having a large EO coefficient along an axis orthogonal to the corresponding optical axis A1or A2and that is aligned with the polarization of incident light pulse P0. Example crystals include lithium niobate (Li2NO3O) as well as crystals such as BaTiO3, KNbO3, and KTa0.35Nb0.65O3that have relatively large EO coefficients and that would keep the applied voltages relatively low. EO units328A and328B are electrically connected to electronics unit30via electrical connection42and electrical lines42A and42B, respectively, included therein. In this EO-based embodiment, control electronics unit38is an EO-unit voltage source, as described below.

In the operation of EO autocorrelation optical system20, linearly-polarized incident light pulses P0having an associated light beam path B0encounters first 50-50 beamsplitter304A. Beamsplitter304A splits each incident light pulse into two co-polarized incident light pulses PAand PB(that form corresponding beams BAand BB) that travel over corresponding interferometer arms320A and320B. Light pulse P1travels through EO unit328A and the at least one EO element330A therein, while light pulse P2travels through EO unit328B and the at least one EO element330B therein. An electrical signal S42Afrom EO-unit voltage source38provides a voltage VAthat can vary to change the effective optical path length OPLAof interferometer arm320A for each incident light pulse P0via a change in refractive index of the at least one EO element330A. Likewise, an electrical signal S42Bfrom EO-unit voltage source38provides a voltage VBthat can to change the effective optical path length OPLBof interferometer arm320B for each incident light pulse P0via a change in the refractive index of the at least one EO element330B therein.

Assuming that when an intermediate voltage V1is provided to EO units328A and328B that the optical path lengths OPLAand OPLBare the same, voltages VAand VBcan be varied (say, with VB<VIand VA>VI) in order to sweep through varying values for the optical path length differences ΔOPL, which allows for the sweeping of pulses PAand PBto perform the autocorrelation of input pulse P0based on a number of substantially similar input pulses P0. Thus, pulses PAand PBexit respective EO units328A and328B with different delays and are combined (interfered) at 50-50 beamsplitter304B. The interfered pulses PAand PBassociated with each input pulses P0form an autocorrelation interference pattern that sweeps over LED detector32, which converts this pattern into autocorrelation signal SA. Autocorrelation signal SA(that is to say, the raw autocorrelation signal SAR) is then processed as described below to obtain a measurement of the pulse length τ0.

Rather than relying on prism movements to provide the phase delay between the two interfered optical beams for each inputted light pulse P0, the EO autocorrelator optical system20of the present invention relies on varying the voltage applied to the EO units328A and328B by EO-unit voltage source38. In an example embodiment, each EO unit328A and328B includes multiple EO members330A and330B, respectively, so that a lower voltage is required to achieve the same change in the optical path length difference ΔOPL than is possible with a single EO member that operates at a higher voltage. Further, by providing both a positive path length change in one interferometer arm and a negative path length change in the other interferometer arm, the required voltages provided by signals S42Aand S42Bare further reduced.

The number of EO devices330A and330B used depends on the type of EO members used. If one wants to measure pulses as short as 500 fs, then the total change in the pulse needs to be >1000 fs. For an average EO coefficient of 70×10−12mN, and if each EO member is X mm long, then the required applied voltage per EO member (assuming NEOis the number of EO members in each arm) is given by:
V=2dΔneeded/(NEOΔninduced)
where d is the combined length of all EO members in one interferometer arm, Δnneededis the indices difference needed to sweep over the given range, and Δninducedis the induced change in refractive index on one EO member. Note that Δnneededis calculated from the travel time difference between the two arms.

Assume τAis the time required for light to traverse arm320A and τBthe time required for light to traverse arm320B, the delay introduced is τB−τA=2dΔn/c where Δn=Δnneeded.

The maximum voltage range is governed by the parameters of the particular EO member(s) used. Voltage values can be as high as 10 kV but are typically around 7 KV for most EO members. The minimum range is set by the length of the pulse to be measured. The longer the pulse length, the lower the voltage required.

FIG. 6is a schematic diagram of an optical-fiber-based EO autocorrelation optical system20. System20ofFIG. 6is similar to that ofFIG. 5, except that optical fiber sections360A and360B are used to form interferometer arms320A and320B. The use of optical fiber sections360A and360B eliminates the need for bulk-optics beamsplitters304A and304B, and mirrors306A and306B. Beamsplitters304A and304B are replaced with 3-dB splitters366A and366B, respectively. Also, input pulse P0is carried by an input optical fiber370optically coupled to input 3-dB splitter366A. LED detector32is provided at the output end of 3-dB splitter366B. Optical fiber sections360A and360B are preferably formed from single-mode optical fibers.

FIG. 7is a schematic diagram of an example embodiment of an integrated-optical (IO) autocorrelation optical system20similar to the optical-fiber-based EO autocorrelation optical system ofFIG. 6. The IO EO autocorrelation optical system20ofFIG. 7is formed on a substrate400suitable for supporting optical waveguides. In an example embodiment, substrate400is or includes silicon. In an example embodiment where the waveguides and EO members are grown on substrate400, the substrate is one suited for the growth techniques used. In this regard, silicon processing techniques are well known and understood, and are therefore preferable.

Substrate400includes an input waveguide410having an input end412. IO EO autocorrelator optical system20also includes first and second waveguide arms430A and430B having respective input and output ends432A,434A and432B,434B. In an example embodiment, the waveguides are formed as slab waveguides that are, for example, deposited atop the substrate.

Also included in system20is an input 3 dB splitter450having an input end452and an output end454, and an output 3 dB splitter460having an input end462and an output end464. Waveguide arm input ends432A and432B are optically coupled to the first 3 dB splitter450at its output end454, while waveguide arm output ends434A and434B are optically coupled to the second 3 dB splitter at its input end462, thereby forming an IO MZ interferometer400analogous to the bulk-optics MZ interferometer100ofFIG. 5. Output end414of input waveguide410is optically coupled to input end452of the input 3 dB splitter450. In an example embodiment, a fiber-optic coupler470(which is one form of the aforementioned optical coupling system24) is provided at input end412of input waveguide410to allow for input optical fiber370(also preferably having a coupler470) to be optically coupled to the input waveguide. The output end464of output 3 dB splitter is optically coupled to LED detector32, which in an example embodiment is integrated with substrate400.

IO EO autocorrelator optical system20includes EO units328A and328B that respectively include at least one EO member330A and330B. In an example embodiment, EO members330A and330B are lithium niobate EO modulators.

The operation of IO EO autocorrelation optical system20ofFIG. 7is analogous to that of the EO autocorrelation optical systems20ofFIG. 5andFIG. 6. Each input pulse P0traveling on input optical fiber370passes to input waveguide410. Pulse P0is then split by 3 dB splitter450, thereby forming two equal-intensity pulses PAand PB. Pulse PAtravels over waveguide arm430A, while pulse PBtravels over waveguide arm430B. EO units328A and328B and the respective EO members430A and430B therein are activated via respective variable voltage signals S42A and S42B. This causes a varying optical path length difference ΔOPL for each input pulse P0, which allows for the sweeping of the relative delay of pulses PAand PBto form the autocorrelation interference pattern representing the autocorrelation of pulse P0. Pulses PAand PBencounter output 3 dB splitter460, where the pulses are combined (outputted) and detected by LED detector32at the output end264of the output 3 dB splitter. LED detector32then generates autocorrelation signal SA, which is processed by electronics unit30as described below.

By way of example, for waveguides430A and430B having a width of 50 microns, and ten EO members330A and ten EO members330B, the IO EO measurement system10is able to perform pulse-width measurements on pulse as short as 400 fs. Shorter pulses can be measured by increasing the number of EO members in EO units328A and/or328B, and/or by increasing voltages VAand/or VB.

IO EO autocorrelator optical system20can be made quite small, which in turn makes the overall measurement system10very compact. In the case where system20is fabricated using a semiconductor substrate, then the limiting size factors are how close EO members330A and330B can be placed to one another, and how close the electrodes can be laid to avoid arcing. Assuming the EO members can be placed in a reasonable amount of space, system20could be made as small as a few inches by a few inches. The use of coupling optics24at the input end of system20would make the device somewhat larger but not significantly so. In an example embodiment, control electronics unit30(and optionally power supply44) are housed in a electronics housing HEseparate from autocorrelation optical system20. Electronics housing HEneed not be very large because the system does not use high voltage.

d) Electronics Unit

FIG. 8is a detailed schematic diagram of an example embodiment of electronics unit30suitable for use with the various embodiments of EO-based autocorrelation optical system20as described above. Electronics unit30includes most of the same elements as described above in connection with the prism-based measurement system10, so that only the differences with the above-described electronics unit are discussed.

As discussed above, control electronics unit38, rather than being a prism driver38, is now a EO-unit voltage source38adapted to provide variable voltage signals S42Aand S42Bto EO units328A and328B, respectively. EO-unit voltage source38is electrically coupled to microcontroller270, as is user input device230.

The operation of electronics unit30is also similar to that described above in connection with prism-based measurement system10, so that only the differences are described here. The main difference is that microcontroller270is adapted (e.g., programmed) to control EO-voltage source38so that it provides the necessary voltages VAand VBfor sweeping ΔOPL to perform the autocorrelation of input pulse P0. Rather than recording the position of prism stage70, microcontroller270records the values for voltages VAand VBprovided by EO-unit voltage source38over a range of voltage values, and correlates these values with the corresponding values for ΔOPL. Microcontroller270is adapted (e.g., programmed) to calculate the pulse length τ0of input pulse P0based on the voltage values (or the value of ΔOPL), and the number of peaks NCin signal SAas represented by digital count signal SC. This calculation is described below. After each sweep of ΔOPL over a select range, microcontroller270generates a calculated pulse length signal SPLrepresentative of pulse length τ0and sends this signal to display40to display the calculated pulse length. Microcontroller270also sends a reset signal SRESto digital logic counter260that resets the digital logic counter to zero after each sweep over the select range of ΔOPL so that the next input pulse P0can be measured.

e) Pulse Length Calculation

To calculate the pulse length τ0, the values for ΔOPL must be known. In an example embodiment, an estimate of the range for pulse length τ0is inputted (e.g., via user input unit230) into microcontroller270. This narrows the needed range for voltages VAand VB, which in turn narrows the range of autocorrelation signal SA.

For short pulses P0, only a ΔOPL is required because the autocorrelation trace is small. By making ΔOPL greater than necessary, the possibility of reading noise increases.

Consider a Gaussian linearly chirped input pulse having the following expression:
E=exp(−(1+ia)(t/τG)2)
where a is the linear chirp parameter and τGis the width of the pulse. The autocorrelation, as detected by the detector is given by:
G2(τ)=1+2exp(−(t/τG)2)+4(exp(−(a2+3)/4*(t/τG)2))cos(a/2*(t/τG)2)*cos(ωτ)+2(exp(−(1+a2)(t/τG)2))cos(2ωτ)
(from Diels “Ultrashort laser pulse phenomena”), where ω refers to the variation of the function G2(τ) as a function of its argument τ. The pulse measurement is obtained by the method described for the prism setup. Count the number of peaks and retrieve the pulse duration from there. Notice that τ in that case is a function of voltage: τ=(2d/c)*(1/2n3rV/l), where c is the speed of light, r is the EO coefficient, n is the refractive index of the EO member, l is the width of the EO member as seen by the applied electric field, d is the length of the EO modulator seen by the light, and V is the applied voltage.
IV. Pulse-Width Accuracy

The accuracy of light-pulse measurement system10depends on how close the reference level LTHfollows the FWHM of the autocorrelation. If the reference level LTHis higher than the FWHM level, then the measured pulse length will be shorter than the actual, and vice versa for lower reference levels. In an example embodiment, the reference FWHM level is set by hand using an oscilloscope to measure the autocorrelation trace. This can be accomplished easily with electronics by finding the peak value and the base of the trace, dividing by two, and setting the reference level at that voltage.

Aside from the FWHM level, there will sometimes be a small deviation of one or two fringes in signal SAbetween pulses P0. This small amount of shifting is equivalent to about 3 femtoseconds, which can be due to the actual differences in light pulses P0due to the light source22rather than measurement system10. For most applications, this amount of error is negligible. The EO-based measurement system and the prism-based measurement system of the present invention will generally have similar accuracy since the system accuracy determined mainly by the measurement electronics.

V. System Alignment

The repeatability of prism-based measurement system10is as good as any other similar commercial measurement system. Every time the system is moved, the input laser beam needs to be re-aligned to LED detector32. Depending on the quality of the alignment, this can affect the performance of the system. System10preferably has at least two pre-set alignment points (e.g., a set of irises) that when aligned, ensure that the system makes accurate measurements. Ultimately, overall repeatability and accuracy is up to the end user's ability to align the system. The prism-based measurement system of the present invention, however, is much easier to align than other commercial systems.

The prism-based measurement system of the present invention is simple to align, beam-shape independent, and inexpensive to manufacture. The most expensive item needed for its construction is the Wollaston prism, which typically ranges from $500-1000. Each prism has a specific pulse length range. A prism with a deviation angle θ=15° can measure pulses from 10 fs to 500 fs. Most measurements systems, such as the FROG system mentioned above, cannot cover this wide of a range. In addition, by changing the prism, the measurement range can be changed. Thus, an example embodiment of the prism-based measurement system includes having two or more prisms with different deviation angles inside the unit. The prisms are then automatically or manually placed into the operating position as needed, depending on the required measurement range.

A preferred embodiment of the EO-based measurement system of the present invention is the integrated optical (IO) embodiment. The IO embodiment does not need to be aligned after its construction. Since it is waveguide based, there is preferably an optical system adapted to couple the light pulse into the waveguide. Accordingly, system alignment is straightforward as compared to the prism-based measurement system.

COMMERCIAL UTILITY

The systems and methods of the present invention should find great commercial utility in quickly and inexpensively measuring ultra-short light pulses for a variety of scientific and industrial applications that require characterizing an otherwise uncharacterized or uncertain output (i.e., the pulse length) of a laser light source.