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Mode Selection, Stabilization and Coupling of Thin Film Lasers by Forward Bragg Diffraction of Periodic Structures

IP.com Disclosure Number: IPCOM000080597D
Original Publication Date: 1974-Jan-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 6 page(s) / 96K

Publishing Venue

IBM

Related People

Lean, EG: AUTHOR

Abstract

Laser oscillation by distributed feedback using periodic structures in thin film lasers, has been proposed and experimentally observed in dye laser configurations. The distributed feedback is provided by backward Bragg diffraction from a periodic structure, which can be a periodic spatial variation of the refraction index or the gain constant in the laser medium, or the periodic variation in the boundaries of the thin film lasers. This type of distributed feedback operates under the Bragg condition given by 2 Beta = mK (1) m = 1, 2, 3 ---. where Beta = 2Pi/Lambda g is the propagation constant of the lasing mode. K = 2Pi /d is the corresponding wave vector of the periodical structure with a periodicity d. Equation (1) can be written as d = m(Lambda g Over 2 (2). For lasers in visible region Lambda G Approximately 0.

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Mode Selection, Stabilization and Coupling of Thin Film Lasers by Forward Bragg Diffraction of Periodic Structures

Laser oscillation by distributed feedback using periodic structures in thin film lasers, has been proposed and experimentally observed in dye laser configurations. The distributed feedback is provided by backward Bragg diffraction from a periodic structure, which can be a periodic spatial variation of the refraction index or the gain constant in the laser medium, or the periodic variation in the boundaries of the thin film lasers. This type of distributed feedback operates under the Bragg condition given by 2 Beta = mK (1) m = 1, 2, 3 ---. where Beta = 2Pi/Lambda g is the propagation constant of the lasing mode. K = 2Pi /d is the corresponding wave vector of the periodical structure with a periodicity d. Equation
(1) can be written as d = m(Lambda g Over 2 (2). For lasers in visible region Lambda G Approximately 0.3Mu m inside the laser medium, this provides a difficult and stringent requirement in fabricating the periodic structure with d Approximately 0.15Mu m (for m = 1).

Described is how a forward Bragg diffraction is used to couple two propagating lasing modes by a periodic structure, together with end mirrors for the feedback. This type of coupled mode thin film lasers provide stable single- mode output, and can have the advantage of output coupling by the periodic grating.

One of the device configurations is shown in Fig. 1. The thin film laser medium supports at least two propagating modes, which can be both TE or TM or one TE and one TM mode. The propagation constants of these modes are defined as Beta (1) and Beta (2). A periodical structure with a periodicity d in the laser medium is also shown in Fig. 1. The periodic structure can be in the index of refraction, in the gain constant, in the boundary of the thin film guide or an acoustic wave propagating along the lasing direction. Under the condition that Beta (1) - Beta (2) = 2 (Pi) Over d (3) the energy of one mode will couple into the other mode.

The periodicity d under the condition (3) is given by d = Lambda (1) Lambda
(2) Over Delta Lambda where Lambda (1) and Lambda (2) are wavelengths of two lasing modes, Delta Lambda = Lambda (1). Depending on the value of Delta Lambda, d can be a few Mu m up to 100 Mu m. The structure with the periodicity larger than Mu m is much easier to fabricate.

Fig. 2 shows the typical field distribution of the two modes (propagating in the same direction) as a function of distance. The distance L of the total energy transfer depends on the coupling constancy of the periodic structure. As shown in Fig. 2, the mode one has a maximum field intensity E(1) (0) = max and E(2) (0) = 0 at x=0, while E(1) (L) = 0 and E(2) (L) = max at x = L. This type of mode conversion by acoustic surface waves has been observed in the prior art. Mode Selection

1

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By placing the mirrors at x = 0 and x = 1, to reflect only

E(1) and E(2),...