Vocabulary Practice

1. Combine the words from groups a and в to make terms:

A B

1) brewster	6) remote	a) stability	f) amplifier
2) steady-state	7) absorber	b) sensing	g) angle
3) environmental	8) pulse-to-pulse	c) method	h) effects
4) shot-to-shot	9) matrix	d) train	i) passes
5) successive	10) pulse	e) separation	j) cell

2. Fill in the blank spaces with the word-combinations that follow:

in essence – в сущности, по существу; in comparison with – по сравнению; in general – в общем, вообще; in order to - (для того) чтобы; in addition (to) – кроме того.

1. __________ produce long pulse train it is required to obtain an ultralong cavity.

2. ___________the above mentioned methods there exist many other techniques for mode selection.

3. The effective cavity length of the compact mode-locked laser _________ the conventional one is from 5 to 21 m.

4. In some cases the oscillator can act as an amplifier to the optical pulse which__________ overcomes additional losses in the cavity.

5. Such oscillator could ___________be referred to as a steady-state amplifier.

TEXT 12A COMPACT PICOSECOND Nd: GLASS MODE-LOCKED

LASER WITH VARIABLE CAVITY LENGTH

I. Introduction

For over 20 years solid-state mode-locked lasers have been the workhorse for investigating picosecond and nonlinear optical phenomena. A typical solid-state mode-locked laser such as Nd:glass produces a pulse train lasting for 100ns of high peak power (2-GW) pulses of 8 ps duration. In a conventional mode-locked laser the cavity length ranges from I to 2 m due to available limited space and mechanica1 stability. The pulse separation in such a conventional mode-locked laser is between 5 and I0 ns. To determine the time dependence of phenomena occurring in materials without interference from other exciting pulses in the pulse train a single pulse is usually extracted from the train of pulses produced by the laser using an electro-optic device. For certain situations it is desirable to have an ultralong pulse-to-pulse spacing for applications such as ranging and remote sensing to mention a few. Long pulse-to-pulse separation is important to reduce the effect of gain depletion and shot-to-shot optical excitation of samples. To produce such a pulse train requires an ultralong cavity. The folding of a cavity reduces the overall length.

This text reports on a new compact mode-locked laser design with an effective cavity length from 5 to 2I m. The per-formance and characteristics of the new laser such as parameters of energy, number of pulses, length of train and stability of the cavity are presented.

II. Experimental Setup

A diagram of the compact design is shown in Fig.3 Brewster angle cut neodymium doped silicate rod and saturable absorber cell is located between several mirrors making up the cavity. The overall geometrical length of the laser is less than 1.7 by 0.3 m wide. The cavity contains a rear mirror (M₄) with a radius of curvature of I.5 m, three spherical optical path extender mirrors (M₁ – M₃), each with a radius of curvature of 1 m, and an output mirror (M₀). The rear mirror and optical path extender mirrors were all coated for 100% reflectivity at 1.06 m. The output mirror (M₀) was coated for 65% reflectivity. The mode-locking dye cell was filled with Kodak 9860 dye dissol-ved in 1.2-dichloroethane with transmission set at 70%.

The pulse train length and spacing were measured using an ultrafast Hamamatsu (CI083) PIN diode coupled to a Tektronix 519 oscilloscope. The pulse duration was measured with a Hamamatsu C979X streak camera coupled to a video analyzer. The pulse train energy for each cavity length was measured using a Laser Precision Energy meter (RK-3230). Shot-to-shot stability of the laser appeared to be excellent for all cavity spacings.

The three optical path extender mirrors (M₁-M₃) were placed between the back mirror (M₄) and the dye cell. This provided a means of varying the optical path length of the cavity without changing its geometrical length. The distance between the mir-rors was as follows: M₀ – M₁ was set at 1.7 m, M₁ – M₂ and M₂ – M₃ were set at 1 m each, and M₃ – M₄ was set at 1.65 m.

III. Description of Cavity Design

To describe how the beam profile changes within the cavity a сingle pass through the path extender section is considered for simplicity. A collimated beam originating at the laser rod reflects off mirror M₁ and focuses to the midpoint (at 0.5 m) between mirrors M₁ and M₂. Mirror M₂ then recollimates the beam, then travels and reflects off mirror M₃. Mirror M₃ then focuses the beam to a point 0.5 m away which is located at the radius of curvature of mirror M₄. The beam in turn reflects off M₄ and refocuses at the same point located 1.5 m from mirror M₄. The beam then retraces its path back through the cavity.

The transverse structure appears to be inform due to the inherent refocusing and recollimating properties of the M₁, M₂, M₃, and M₄ mirror arrangement and the location of the apertures.

By adjusting mirror M₃ the cavity length can be changed in 4-m steps corresponding to 13 ns/step. The travel time for a pulse through the beam path extender section for N reflections off the central mirror M₂ is Т = (2N + 2) L/c, where L is the distance between mirrors M₁ – M₂ and M₂ –M₃ .The path lengths were measured from the pulse trains to be 5.25, 9.25, 13.25, 17.25 and 21.25 m.

IV. Experimental Results

Laser thresholds, pulse-to-pulse time spacing and pulse energies with respect to the number of passes through the path extender mirrors and the effective optical path length are sum-marized in Table 1. The duration of a pulse early in the train is 10.5 ps.

For various passes through the path extender section of the cavity the threshold energy increased slightly from 3.2 to 4.7 kV while the output energy remained fairly constant at 550mJ with the exception of nine passes (N = 9). For N = 9 passes one of the reflections ran off the edge of the central mirror (M₂) resulting in a decrease in energy at the output.

The pulse train length was found to be extremely stable and long, of the order of 5 μs. This was attributed to the high stability of the cavity and long pulse separation, which allows the lasing medium ample time to recover and repopulate between successive passes of the optical pulse. In this case, the oscil-lator appears as an amplifier to the optical pulse, which in turn overcomes additional losses in the cavity due to multiple reflections off each mirror. This may be viewed in essence as a steady-state amplifier.

V. Applications

Pulse-to-pulse spacings can easily be selected in steps of I3.2 ns with the realignment of only one mirror (M₃), no other mirror is required to be realigned. Fоr longer (or shorter) pulse separations this design can be scaled. With this cavity design we achieved a long optical path length in a short geometrical length (1.7 m) thereby minimizing environmental and mechanical effects providing a very stable configuration. The physical size, stability, and elimination of devices external to the cavity are clear advantages over the standard two-mirror cavity designs. Applications may include dynamic remote sensing and ranging of objects through the modulation of the pulse train.

VI. Summary

In summary, the overall size of the new laser design is 1.7 m long by 0.3 m wide. The pulse-to-pulse spacing is variable from ≈ 35 to I40 ns. The train length was of the order of 5 with ≈100 pulses. The pulse duration of early pulses was mea-sured to be 10.5 ps. The stability diagram for the cavity was calculated using the matrix method.

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Review

3. Match the parts of sentences in column A with those in column B to produce true statements:

The cavity length of a conventional mode-locked laser	a rear mirror, three optical path extender mirrors and output mirror.
2) The transverse structure appears to be uniform due to	with a Hamamatsu camera coupled to a video analyzer.
3) The cavity contains	using the matrix method.
4) The pulse duration was measured	to recover and repopulate.
5) The stability zone for the cavity is calculated	dynamic remote sensing and ranging of objects.
6) The high stability of the cavity and long pulse separation allows the lasing time	the inherent refocusing and recollimating properties of the mirrors.
7) As for applications, they may include	ranges from I to 2m.

4. Pick out the words that do belong the following groups:

1. refocus, recollimate, retrace, remain, repopulate, realign, reconvert;

2. pulses, trains, results, samples, ranges, radians;

3. integration, cooperation, combination, unity, separation.

5. Study the following sentences and reorder them according to the text to obtain a logical abstract

4. The overall size of the laser is 1.7 m.

5. The stability zone for the cavity is calculated using the matrix method.

6. The design, performance and characteristics of a compact mode-locked laser with effective cavity length of 5 to 21 m is described.

7. The pulse duration was measured to be 10 ps for the early pulses in the train using a streak camera system.

8. Pulse-to-pulse spacing of ≈ 35 =140 ns is obtained.

9. The laser parameters of energy, number of pulses, length of train and stability of the cavity are described.