DISPERSIVE ELEMENT FOR OPTICAL PULSE COMPRESSION

• US 3,679,313 A
• Filed: 10/23/1970
• Issued: 07/25/1972
• Est. Priority Date: 10/23/1970
• Status: Expired due to Term

• Current Assignee: Robert Rosenberg
• Original Assignee: Robert Rosenberg
• Inventors: Rosenberg, Robert
• Primary Examiner(s): Wibert, Ronald L.
• Assistant Examiner(s): Clark, Conrad
• Application Number:
  US05/083,414
• Time in Patent Office: 641 Days
• Field of Search: 331/94.5 356/106,113 350/160
• US Class Current: 356/519
• CPC Class Codes: H01S 3/0057 Temporal shaping, e.g. pulse compression, frequency chirping soliton generation and propagation G02F1/3513, H01S3/063 and H01S3/108

Abstract

An interferometer cavity defined by two mirrors of different reflectance is advantageously adjusted to achieve superior compression of optical pulse signals. In particular, a mirror of predetermined partial reflectance is juxtaposed with a mirror of substantially total reflectance, the separation distance being detuned by a calculated amount from a condition of resonance with the optical pulse input. The detuning distance is calculated in terms of the physical characteristics of the cavity and certain delimiting characteristics of the optical pulse signals.

Claims

1. In a system which utilizes electromagnetic signals in the optical frequency range, the signals being characterized by the relation E(t) A(t) ei (t) i being the imaginary root square root -1, the signals E(t) being restricted such that a. the time derivative of the function psi (t) is linear in the intervals in which the amplitude function A(t) is large and b. the frequency spectrum of the signal E(t) comprises a series of equally spaced discrete monochromatic components, interferometer means having a periodic time delay vs. frequency characteristic for subjecting the signals to a frequency dependent time delay comprising:

a cavity defined by first and second reflecting surfaces, said first reflecting surface having partial reflectance and said second reflecting surface having substantially total reflectance, the signals to be delayed being introduced through said first reflecting surface and the delayed signals being emitted from said first reflecting surface, the separation between said reflecting surfaces being adjusted such that a. no period of the periodic time delay vs. frequency characteristic of said cavity contains more than one spectral component of the frequency spectrum of E(t), b. the period of the time delay vs. frequency characteristic of said cavity is different from the separation of the spectral lines of E(t), c. the spectral lines of E(t) occur at substantially linear portions of the time delay vs. frequency characteristic of said cavity, said reflecting surfaces being oriented relative to one another and to said signals to be delayed such that all signals in said cavity spatially overlap one another.

2. Interferometer means as defined in claim 1 wherein said first and second reflecting surfaces each comprise curved surface portions of spherical mirrors, the concavities of said mirrors facing one another, said signals to be delayed being introduced normally to said first reflecting surface.

3. Apparatus for compressing electromagnetic pulses in time comprising:

a source of electromagnetic pulse signals, said electromagnetic pulse signals being characterized by the relation E(t) A(t) ei (t) and being restricted such that a. the time derivative of psi (t) is linear in the intervals in which A(t) is large, and b. the frequency spectrum of E(t) comprises a series of equally spaced discrete monochromatic components;

an interferometer cavity means having first and second reflecting surfaces, said first reflecting surfacE having partial reflectance and said second reflecting surface having total reflectance;

means for coupling the electromagnetic pulse signals from said source in a line normal to said first reflecting surface and for coupling to an output compressed pulses emitted from said first reflecting surface, the separation between said reflecting surfaces being adjusted such that a. no period of the time delay vs. frequency characteristic of said cavity contains more than one spectral component of the frequency spectrum of E(t), b. the period of the time delay vs. frequency characteristic of said cavity is different from the separation of the spectral lines of E(t), and c. the spectral lines of E(t) occur at linear portions of the time delay vs. frequency characteristic of said cavity;

said reflecting surfaces being oriented relative to one another and to said electromagnetic pulse signals such that all signals within said cavity means spatially overlap one another, each compressed pulse including the superposition of portions of a plurality of different pulses from said electromagnetic pulse signals.

4. Apparatus for compressing electromagnetic pulses as defined in claim 3 wherein said source of optical pulse signals comprises a mode-locked laser, modulating means for synchronously modulating the output light from said laser, the signals from said modulating means being a train of electromagnetic pulses, each of the pulses of said train having a linear frequency variation in time;

and said means for coupling includes mode matching lenses for grouping in space similar frequency components of the pulses from said modulating means, and directional coupler means for applying the pulses from said modulating means to said mode-matching lenses.

5. Apparatus for compressing electromagnetic pulses as defined in claim 3 wherein said source of electromagnetic pulse signals comprises a single frequency laser, a sinusoidal phase modulator for modulating the output light from said laser;

and said means for coupling includes mode matching lenses for grouping in space similar frequency components of the pulses from said phase modulator and means for applying the pulses from said modulating means to said mode matching lenses.

6. Interferometer apparatus for providing a frequency dependent time delay to an optical signal whose frequency spectrum consists of a series of substantially discrete monochromatic components equally spaced at an interval of F Hertz, said interferometer apparatus comprising a cavity having first and second reflecting surfaces separated by a distance L and having a fundamental resonant frequency of fR, said first reflecting surface having partial reflectance and said second reflecting surface having substantially total reflectance, signals to be delayed being introduced normally to said first reflecting surface and delayed signals being emitted from said first reflecting surface, said distance L being defined by the relation where c is a constant equal to the speed of light, N is an integer equal to or greater than unity, and x is a mismatch factor defined by the relation said reflecting surfaces being oriented relative to one another and to said signals to be delayed such that all signals in said cavity spatially overlap one another.

7. Apparatus for compressing electromagnetic pulses in time comprising a source of electromagnetic pulse signals that are characterized by the relation E(t) A(t) ei (t) where i is Square Root -1, and the signal E(t) is restricted such that (a) the time derivative of the function psi (t) is linear in the intervals where the amplitude function A(t) is large, and (b) the frequency spectrum of E(t) consists of a series of substantially discrete monochromatic components equAlly spaced at an interval of F Hertz, an interferometer cavity means defined by first and second reflecting, surfaces separated by a distance L and having a fundamental resonant frequency of fR, said first reflecting surface having partial reflectance and said second reflecting surface having substantially total reflectance, means for coupling the electromagnetic pulse signals from said source in a line normal to said first reflecting surface and for coupling to an output the delayed signals being emitted from said first reflecting surface, the distance L being defined by the equation where c is a constant equal to the speed of light, N is an integer equal to or greater than unity, and x is a mismatch factor defined by the equation said reflecting surfaces being oriented relative to one another such that all signals in said cavity spatially overlap one another, each compressed pulse including the superposition of portions of different pulses to be compressed.

8. Apparatus for compressing electromagnetic pulses as defined in claim 7 wherein said first and second reflecting surfaces each consist of a curved surface portion of a spherical mirror, and said first and second reflecting surfaces are arranged such that the concavity of said first reflecting surface faces the concavity of said second reflecting surface.

9. In a system having pulsed signals characterized by a periodic comb-type frequency spectrum, each pulse having a linear frequency sweep as a function of time, and an interferometer cavity defined by first and second reflecting surfaces and having a periodic time delay versus frequency characteristic, said first reflecting surface having partial reflectance and said second reflecting surface having total reflectance, a method of compressing said pulses comprising the steps of:

applying said input pulses normally to said first reflecting surface;

adjusting the orientation of said reflecting surfaces relative to one another and to said input pulses such that consecutive reflections in said cavity spatially overlap one another; and

adjusting the separation of said reflecting surfaces such that spectral lines of said comb-type frequency spectrum occur at linear portions of the time delay versus frequency characteristic of said cavity, and that said time delay versus frequency characteristic of said cavity has a periodicity smaller than the periodicity of said comb-type frequency spectrum.

10. In a system having pulsed signals characterized by a periodic comb-type frequency spectrum consisting of a series of substantially discrete monochromatic components equally spaced at an interval of F Hertz, each pulse of said pulsed signals having a linear frequency sweep as a function of time, and an interferometer cavity defined by first and second reflecting surfaces, said first reflecting surface having partial reflectance and said second reflecting surface having total reflectance, a method of compressing said pulses comprising the steps of applying said pulsed signals in a line normal to said first reflecting surface, adjusting the orientation of said reflecting surfaces relative to one another and to said pulsed signals such that consecutive reflections in said cavity spatially overlap one another, and adjusting the separation of said reflecting surfaces to a distance of L where L is defined by the equation where c is a constant equal to the speed of light, N is integer equal to or greater than unity, and x is a mismatch factor defined by the equation where fR is the fundamental resonant frequency of said cavity.