Optical: systems and elements – Optical frequency converter
Reexamination Certificate
2001-12-07
2003-06-03
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
C359S199200, C372S025000
Reexamination Certificate
active
06574036
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention concerns a method and a device for the programmable time profile shaping of quasi-monochromatic optical pulses.
2. State of the Prior Art
In certain applications that use pulsed laser sources, it is necessary to shape the time profile of the amplitude of the optical pulses. Examples that may be cited include the field of power lasers, or the telecommunications field, where it is necessary to shape the profile of pulses in the time domain before transmitting information.
Procedures for shaping the time profile of optical pulses aim to satisfy several criteria simultaneously:
To obtain time profile shaping with good resolution, with preferably 100 to 1,000 points over the whole length of the pulse, with each point being able to reach the femtosecond domain.
To be programmable, in other words to allow a change in the shape of the pulse both when desired and rapidly, for example in an automatic manner.
To be compatible with the production of pulses that have spectra that are as narrow as the Fourier limit of the created pulses.
To produce pulses that can be wavelength tuned.
To not cause too much energy loss.
Basically, two general approaches are used to shape the time profile of laser pulses. The first uses essentially optical methods. The second consists in converting the electronic signals into optical signals, with the corresponding procedures using electro-optical or acousto-optical systems.
Optical Procedures
Procedure For Time Shaping by Spectral Modulation
An entirely optical modulation procedure of this type is described in the document reference (1) at the end of the description. In this procedure, as shown in
FIG. 1
, a beam of light
10
from a source point
0
1
, which is collimated, is diffracted by an optical grating
11
so that it can then be focused at point
0
2
. The pupil
12
is located in plane P of the grating
11
. When a short pulse is diffracted through the pupil in a given direction, it is possible to construct a time profile h (t) by applying a primitive function of h (t) as a transmitting function using the mask
13
, in the plane of the pupil
12
following the direction x. A spatial filter
14
is placed at the point
0
2
in order to obtain a spectrally homogeneous beam. Such a device is similar to a spectrometer. The time resolution is identical to the length of the injected short pulse. The length of the output pulse is equal to the relative maximum time delay of each of the rays of the beam that covers the pupil
12
in the plane P.
This procedure enables good shaping performance of the time profile to be obtained, particularly as regards the number of desired points. It is programmable: a variable spatial filter only has to be placed in the plane of the pupil. It produces quasi-monochromatic pulses. In addition, it allows a tuneable wavelength to be produced. On the other hand, it has very low energy yield, around the inverse of the number of resolved points.
Procedure For Time Shaping by Fourier Transform
The fields in the frequency range and in the time range are linked by the Fourier transform E(v)=TF[E (t)].
If one wishes the time profile A (t) of the pulse to lie within the Fourier limits, the field must posses a linear phase with the spectral variable. The pulse is then quasi-monochromatic. By acting on the amplitude and the incident pulse phase, it is possible to modify its time profile. The spectral mask M (v) in amplitude and in phase must then satisfy the equation: XXX, where X is the incident spectral field of the device. The procedure therefore consists in simple spectral selection (in amplitude and in phase), providing that the incident spectral field has all of the spectral components of the field to produce (in other words, if (M(v))<1).
In the device shown in
FIG. 2
, initially one has a short pulse with a wide spectrum. The spectral amplitude is modified using a spectral selection device
20
, made out of spectral amplitude and phase filters. A time profile is then obtained, which is the Fourier transform of the spectral amplitude which has been shaped in the spectral plane.
It can be seen that the spectral selection is achieved in a similar manner for the majority of optical systems. One only has to place a spatial filter in a specific plane of the spectral selection device. It is then possible to achieve programmable shaping by using a variable transmission spatial system.
This Fourier transform time shaping procedure gives goods results as regards the number of resolved points since the desired resolutions may be obtained. It is also programmable, since it uses spectral selection. This procedure also enables a narrow spectrum to be generated, and the wavelength is, in addition, tuneable: one only has to displace the mask in the spectral plane in order to change the wavelength. Although the pulse length is variable, it is dependent on the spectral width of the pulse produced (via the intermediary of the Fourier transform). It is not therefore possible to obtain long pulses. The main disadvantage of this type of procedure is, in fact, the energy yield. As a matter of fact, time shaping with good resolution in the spectral plane requires considerable losses in energy if a narrow spectrum is desired, since a large part of the spectrum must be cut off. This procedure is, as a result, only used very infrequently.
Procedure For Time Shaping Pulses With Frequency Drift
This type of procedure is described in document (2).
FIG. 3
shows a schematic diagram of this procedure. The pulses are schematically represented in order to show their length and their spectral width. Time shaping by completely optical means may be achieved by using short pulses. Such pulses, which have wide spectra, are frequency drifted after going through a dispersion device
22
, as in that described in document (3). The wavelengths are then time dispersed, which involves spreading out the wavelengths that make up the wide spectrum over time. By selecting the wavelengths using a spectral selection device
23
, time shaping is carried out.
Very good resolution may be achieved using this procedure, with the number of resolved points greater than 100. This procedure allows programmable time shaping: it consists, in effect, in transferring the shaping into the spectral range and a large number of programmable spectral selection systems are available, such as that described in document (4). The energy loss caused by this type of procedure is minimal, since only the essential components of the pulse are lost in time shaping. However, the shaped pulse has a wide spectrum.
It can be seen that procedures that use a spectral selection device
23
to carry out the shaping of the time profile of a pulse, whose means of operation is schematically shown in
FIG. 4
, generally all operate on the basis of a spectrometer. The resolving power is, by definition, the ratio between the central wavelength of the device &lgr; and the spectral resolution &dgr;&lgr;:R=&lgr;/&dgr;&lgr;. The number, P, of time resolved points is defined by T
1
/T
2
and is given by the equation:
P
=
Δ
λ
λ
λ
δ
λ
.
The quantity &Dgr;&lgr;/&lgr; is a characteristic of the incident laser pulse in the system. The order of magnitude of the &Dgr;&lgr;/&lgr; quantities is from 10
−2
to 10
−1
depending on the wavelengths used. The resolving power is conventionally around 10
4
. This type of device therefore allows a number of resolved points of between 100 and 1,000 to be obtained.
Electro-optical Procedures
These procedures achieve time profile shaping by converting the electronic signals into optical signals and obtaining an interference between the non-modified part of an optical impulsion and the modified part of this optical pulse using an electrical signal.
FIG. 5
shows a schematic diagram of this type of procedure. A field E
0
with intensity I
0
is separated into two parts. The two fields E
a
and E
b
follow different paths, and
Husson Daniel
Migus Arnold
Raoult Fabrice
Rouyer Claude
Sauteret Christian
Commissariat a l'Energie Atomique
Hayes & Soloway P.C.
Lee John D.
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