VU3ZAG Expedition: LUNAR LASER COMMUNICATION

INTRODUCTION

The present trends of communication are through transmission lines, optical fiber and microwave wireless communications. Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. Microwave transmission refers to the technology of transmitting information or power by the use of radio waves whose frequencies are spectrum ranges across of roughly 1.0 (GHz) to 30 GHz referred as microwaves. These all type of communication technologies has their own disadvantages. In optical fiber communication losses like attenuation, absorption, scattering and radiative are occurred. Losses can occur in transmission lines if it shouldn’t perfectly terminated at its characteristic impedance. Microwave devices such as MASER can cause ozone depletion due to its high area of radiation.

A new concept of a Free-Space Optical Communication (FSOC) system is introduced. It’s an optical communication technology that uses laser light propagating in free space to transmit data for telecommunication. The technology is useful where the physical connections are impractical due to high costs or other considerations. Here the laser communication is used to transfer information from lunar satellite to earth base station with a compatibility of analog and digital data. It uses a laser diode of wavelength 650nm. Embedded system based transmitter and receiver are designed using PIC microcontrollers which uses NRZ sequences for transmission with ASK modulation which operates laser in pulsed mode.

The main advantages of Laser communication is that it can provide a long distance communication having high speed data transfer with higher bit rates and low bit error rates due to the start and stop bit used for data transfer with high efficiency. Also it provides immunity to electromagnetic interference.

BLOCK DIAGRAM

Block Diagram of Lunar Laser Communication

MICROPHONE

The microphone is a transducer which converts the sound in form of air pressure to the electrical signal. We are using a condenser microphone for acoustic to electric conversion. Condenser means capacitor, an electronic component which stores energy in the form of an electrostatic field. The term condenser is actually obsolete but has stuck as the name for this type of microphone, which uses a capacitor to convert acoustical energy into electrical energy.

Condenser Microphone

But in actual condenser microphones available in the market the capacitance principle is no longer used. Instead they are using an NPN transistor whose collector is used as the positive (+) terminal and the emitter is being used as a negative (-) terminal with the base of the same is made in contact to a thin aluminum diaphragm which vibrates when the air pressure due to sound signal occurs.

AMPLIFIER

The amplifier is a circuit which is designed to boost the signal to a number of times in amplitude without changing its frequency parameters. Operational amplifiers are generally being used for the amplification purpose. An operational amplifier ("op-amp") is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. An op-amp produces an output voltage that is typically hundreds of thousands times larger than the voltage difference between its input terminals.

Notation of Op-Amp

MICROCONTROLLER

A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM. Microcontrollers are designed for embedded applications, in contrast to the microprocessors used in personal computers or other general purpose applications.

Microcontrollers are used in automatically controlled products and devices, such as automobile engine control systems, implantable medical devices, remote controls, office machines, appliances, power tools, toys and other embedded systems. By reducing the size and cost compared to a design that uses a separate microprocessor, memory, and input/output devices, microcontrollers make it economical to digitally control even more devices and processes. Mixed signal microcontrollers are common, integrating analog components needed to control non-digital electronic systems.

We are using a PIC 16F877A microcontroller designed and developed by microchip. The PIC16F877A features 256 bytes of EEPROM data memory, self programming, an ICD, 2 Comparators, 8 channels of 10-bit Analog-to-Digital (A/D) converter, 2 capture/compare/PWM functions, the synchronous serial port can be configured as either 3-wire Serial Peripheral Interface (SPI) or the 2-wire Inter- Integrated Circuit (I2C) bus and a Universal Asynchronous Receiver Transmitter (USART). Of the features we are utilizing the UART, ADC and the PWM.

USART ASYNCHRONOUS MODE

In this mode, the USART uses standard non-return-to zero (NRZ) format (one START bit, eight or nine data bits, and one STOP bit). The most common data format is 8-bits. An on-chip, dedicated, 8-bit baud rate generator can be used to derive standard baud rate frequencies from the oscillator. The USART transmits and receives the LSB first.

NON-RETURN-TO-ZERO

In telecommunication, a Non-Return-to-Zero (NRZ) line code is a binary code in which 1's are represented by one significant condition (usually a positive voltage) and 0's are represented by some other significant condition (usually a negative voltage), with no other neutral or rest condition. The pulses have more energy than a RZ code. Unlike RZ, NRZ does not have a rest state. NRZ is not inherently a self-synchronizing code, so some additional synchronization technique (for example a run length limited constraint or a parallel synchronization signal) must be used to avoid bit slip.

Non Return to Zero Sequence

TRANSMITTER CIRCUIT

The transmitter circuit consists of a switching circuit to switch between the analog and digital communication when needed. It also consists of the laser drive which drives the laser in both the analog and digital mode of communication. The current limiter in the circuit protects the laser from over driven which can cause laser blowout.

LASER

A laser is a device that emits light (electromagnetic radiation) through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation. The emitted laser light is notable for its high degree of spatial and temporal coherence, unattainable using other technologies.

Spatial coherence typically is expressed through the output being a narrow beam which is diffraction-limited, often a so-called "pencil beam." Laser beams can be focused to very tiny spots, achieving a very high irradiance. Or they can be launched into a beam of very low divergence in order to concentrate their power at a large distance.

There are different types of laser available in the market like Gas lasers, Chemical lasers, Photonic crystal Lasers etc. of which semiconductor lasers are being used here.

SEMICONDUCTOR LASER

A laser diode is formed by doping a very thin layer on the surface of a crystal wafer. The crystal is doped to produce an n-type region and a p-type region, one above the other, resulting in a p-n junction, or diode.

Laser diodes form a subset of the larger classification of semiconductor p-n junction diodes. Forward electrical bias across the laser diode causes the two species of charge carrier holes and electrons to be "injected" from opposite sides of the p-n junction into the depletion region. Holes are injected from the p-doped, and electrons from the n-doped, semiconductor.

A depletion region, devoid of any charge carriers, forms as a result of the difference in electrical potential between n- and p-type semiconductors wherever they are in physical contact. Due to the use of charge injection in powering most diode lasers, this class of lasers is sometimes termed "injection lasers “or” injection laser diode" (ILD). As diode lasers are semiconductor devices, they may also be classified as semiconductor lasers. Either designation distinguishes diode lasers from solid-state lasers. These photon-emitting semiconductors are the so-called "direct bandgap" semiconductors.

Laser Diode Front View

DETECTOR AND RECEIVER CIRCUIT

The detector used for the laser beam reception are the photon detectors and photon counters which produce the corresponding electrical current in accordance with the beam intensity of the laser beam being transmitted. This is amplified using the amplifier and then switched to analog or digital circuit based on the type of signal being received. If it’s the sound signal then it’s reproduced in the speaker else displayed in the LCD module.

LCD MODULE

A liquid crystal display (LCD) is a flat panel display, electronic visual display, video display that uses the light modulating properties of liquid crystals (LCs). LCs do not emit light directly. They are used in a wide range of applications, including computer monitors, television, instrument panels, aircraft cockpit displays, signage, etc. The most flexible ones use an array of small pixels.

LCD Module

PS/2 KEYBOARD

A computer keyboard usually has a small numeric keypad on the side, in addition to the other number keys on the top, but with a calculator-style arrangement of buttons that allow more efficient entry of numerical data. This number pad (commonly abbreviated to "numpad") is usually positioned on the right side of the keyboard because most people are right-handed.

PS/2 Qwerty Keyboard

The PS/2 connector is a 6-pin Mini-DIN connector used for connecting some keyboards and mice to a PC compatible computer system. The PS/2 designs on keyboard and mouse interfaces are electrically similar and employ the same communication protocol. However, a given system's keyboard and mouse port may not be interchangeable since the two devices use a different set of commands.

PS/2 Connector

DESIGN AND SIMULATION OF LLC

The design of the project is done in embedded platform using the PIC microcontroller for the demonstration purpose. The simulation is carried out in the embedded simulation tool called Protius 7 professional circuit design suit.

PCB DESIGN OF LLC

The PCB is designed using the EAGLE PCB design software. The PCB is manufactured using the home made traditional PCB cooking method.