Autocorrelation of ultrashort electromagnetic pulses

Details Autocorrelation of ultrashort electromagnetic pulses Autocorrelation of ultrashort electromagnetic pulses

1998-08-05

2001-02-27

Kim, Robert (Department: 2877)

Optics: measuring and testing

By light interference

Reexamination Certificate

active

06195167

ABSTRACT:

This invention relates to the autocorrelation of ultrashort electromagnetic pulses, such as pulses obtained from mode-locked lasers, and in particular to the autocorrelation of pulses with a duration of 1 picosecond to 5 femtoseconds.
BACKGROUND TO THE INVENTION
The durations of pulses produced from mode-locked lasers can be as short as a few femtoseconds (1 fs=10
−15
s) and typically have durations of less than 100 fs. The response times of the fastest electronic circuits are thousands of times longer than the duration of these pulses and therefore electronic techniques cannot be used to directly measure pulse durations. The shortest event available for measurement purposes is the pulse itself and this is the basis of optical autocorrelation techniques used for ultrashort pulse measurement.
In the most common autocorrelator arrangement, an input pulse, or parent pulse, passes into a Michelson interferometer which splits the parent pulse into two daughter pulses which are identical in shape and amplitude. The two daughter pulses travel along separate paths in the interferometer, one path being of variable length by use of a reflecting arm with a variable position. The two daughter pulses exit the interferometer overlapped spatially but with a relative temporal delay equivalent to the difference in path lengths travelled by each of the identical daughter pulses.
A two-wave mixing process, such as second-harmonic generation, is then used to obtain a mixing signal between the two daughter pulses. When the path lengths travelled by each daughter pulse are equal, the relative delay between the daughter pulses is zero and the mixing signal is strongest. As the difference in path length of the two daughter pulses increases, the product of the mixing decreases until, for time delays which are a few times longer than the pulse duration, the mixing signal becomes zero or at least insignificant. Therefore by studying how the mixing signal varies in response to changes in path length, a correlation signal can be obtained where width is related to the width, (i.e. duration), of the original input pulse.
Second-order autocorrelation, where the mixing signal varies quadratically with the optical input power, is common in mode-locked laser oscillators, and second-harmonic generation (SHG) has been used successfully to produce high-quality autocorrelation of sub-picosecond duration pulses. In SHG autocorrelation, the fields from each daughter pulse are coupled by a second-harmonic generation process to produce a wave at twice the fundamental frequency. The wave amplitude E
2
is defined by:
E
2
=E
1a
×E
1b
  (1)
where E
1a
is the amplitude of a first daughter pulse, E
1b
is the amplitude of a second daughter pulse, and where both daughter pulses are directly derived from the same input parent pulse.
The second-harmonic intensity therefore varies quadratically with the input power to the Michelson interferometer. In practice, the further pulses which exit the Michelson interferometer are focused into a frequency-doubling crystal and the frequency-doubled light is then detected using a photomultiplier tube. The output voltage from the photomultiplier tube is then recorded as a function of the path difference between the pulses, or equivalently displacement of one of the interferometer arms, to give the autocorrelation signal of the input pulse.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided an autocorrelator of ultrashort electromagnetic pulses, comprising an interferometer for receiving a parent electromagnetic pulse and for deriving two daughter electromagnetic pulses from the parent pulse; an output means coupled to the interferometer to output the daughter pulses; and diode means in communication with the output means to detect the daughter pulses and alter the frequency thereof, so as to produce an electrical output signal which has a quadratic dependence on the power associated with the parent electromagnetic pulse. Thus typically the alteration in frequency is a frequency doubling so that the quadratic power dependence of the electrical output signal on the input power is achieved.
The ultrashort electromagnetic pulses may have a temporal duration ranging from picoseconds (10
−12
s) to femtoseconds (10
−15
s). Thus the range of pulse durations for which the present invention is intended to be used is over the range 10 picoseconds to 5 femtoseconds, and more preferably from 1 picosecond to 5 femtoseconds, and most preferably from 100 femtoseconds to 10 femtoseconds. A defined pulse width is required for the diode means to produce the required quadratic response and therefore there is an upper limit on the duration of the pulses, beyond which significant quadratic power response is not achieved.
The diode means preferably provides the quadratic power response for pulses with wavelengths of certain energies. Therefore where the diode means has an energy bandgap of energy E
g
, the quadratic response is provided for pulse wavelengths of photon energy less than E
g
but greater than E
g
/2.
The diode means may be a semi-conductor device such as a light emitting diode (LED), a photodiode or a laser diode and others that will become apparent to one skilled in the art. Suitable semi-conductor materials are GaAsP, AlGaAs, InGaAs, Ge, and GaAs, which operate as photovoltaic detectors and therefore require no external electrical bias. Use of diodes ensures a broad band response which can range from 780-4000 nm. For example, ranges achieved for GaAsP are 700-1300 nm, AlGaAs are 680-1300 nm, for GaAs 1000-2000 nm (1-2 micron), for a Ge photodiode 1800-3600 nm; and for InGaAs 2-4.9 microns. The bandwidth of the photodiode &Dgr;&lgr;=hc/E
g
and extends to wavelengths approaching E
g
/2. This allows measurement over a large wavelength range without realignment.
The interferometer may be a Michelson interferometer, or the interferometer may instead be a Wollaston prism interferometer. The Wollaston prism interferometer is of particular advantage as it has an optical path that follows one direction only. Furthermore, no reflecting surfaces are required in a Wollaston prism interferometer which reduces energy losses within the autocorrelator.
Preferably a focusing element is provided between the output means and the diode means so that the pulses from the output means are focused onto a defined area of the diode means.
In accordance with another aspect of the invention, there is also provided a method of SHG autocorrelation for ultrashort electromagnetic pulses, the method comprising deriving two daughter electromagnetic pulses from a parent electromagnetic pulse by use of an interferometer, passing the daughter pulses to a diode means with an energy gap of E
g
, detecting in the diode means daughter pulses of photon energy less than E
g
but greater than E
g
/2, and producing an output electrical signal from the diode means which depends quadratically on the incident power associated with the parent electromagnetic pulse.
The use of a diode means in an autocorrelator provides a solid state device with particular advantages over the prior art use of a photomultiplier tube and frequency-doubling crystal. In particular as semi-conductor diodes are relatively cheap and small devices, their use in an autocorrelator provides a particularly compact apparatus. Also diodes are more robust than photomultiplier tubes as they do not need a high voltage supply or a vacuum to operate. Further the photovoltaic nature of diodes is of advantage as they will generate their own electrical current and do not need to be connected to a power supply.
Disadvantages experienced with the prior art use of crystals are avoided. In particular a photodiode is polarisation insensitive, and is broadband so that for broadband laser pulses, i.e. a short pulse with a large frequency range, the diode has the same response over the entire pulse. This is unlike crystals where movement of the crystal is required to detect over the full range.

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