Abstract
The art of imparting information onto a light wave by optical signal modulation is fundamental to all forms of optical communication. Among many schemes, direct modulation of laser diodes stands out as a simple, robust, and cost-effective method. However, the simultaneous changes in intensity, frequency, and phase have prevented its application in the field of secure quantum communication. Here, we propose and experimentally demonstrate a directly phase-modulated light source which overcomes the main disadvantages associated with direct modulation and is suitable for diverse applications such as coherent communications and quantum cryptography. The source separates the tasks of phase preparation and pulse generation between a pair of semiconductor lasers leading to very pure phase states. Moreover, the cavity-enhanced electro-optic effect enables the first example of subvolt half-wave phase modulation at high signal rates. The source is compact, stable, and versatile, and we show its potential to become the standard transmitter for future quantum communication networks based on attenuated laser pulses.
- Received 15 April 2016
DOI:https://doi.org/10.1103/PhysRevX.6.031044
This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Synopsis
Controlling a Laser’s Phase
Published 20 September 2016
A compact scheme can directly modulate the phase of a laser without a bulky external modulator.
See more in Physics
Popular Summary
Direct modulation of laser diodes is a simple, robust, and cost-effective method for imparting information into light waves, which is fundamental to all forms of optical communication. This technique is widely used in data centers across the globe and has found its way into millions of homes in the form of fiber-optical broadband. Despite these considerable successes, direct modulation cannot provide pure phase modulation without affecting other properties of the light, which represents a key drawback that must be overcome for emerging quantum communication applications. One example is quantum key distribution, which is fast becoming a standard technique for exchanging the secret keys that are essential to securing our communication infrastructure.
Here, we introduce a direct modulation source based on an entirely new concept. Our innovation is to separate the phase preparation and light-pulse generation between two compact laser diodes. One laser acts as an electro-optic component to control the phase of the second laser, and the resulting source overcomes all of the main disadvantages associated with direct-modulation schemes while at the same time retaining all of the benefits. Our setup features an exceptionally low subvolt drive voltage, excellent phase stability, and great versatility, making it an attractive choice for many applications. We experimentally demonstrate how this direct modulation source can be used in the demanding field of quantum cryptography, but we envision that the concept will have much broader appeal in many other areas as well.
We expect that our findings will impact the emerging industry of quantum cryptography by enabling a versatile, low-cost, and modulator-free phase transmitter.